Martha, accompanied by Una and Gilly, got into the Hub early on Monday morning. As they took off their outdoor coats and hats on this wintery day they noticed Jack adjusting a valve none of them had ever seen turned before which released water into the pool at the bottom of the Hub.

“Are we getting a swimming pool?” Una asked.

“Not for you, kiddo,” Jack answered. “This water is coming in from the Bay.”

Una shuddered as she thought how cold the water would be on this November morning. Gilly looked at it with complete disdain and headed for the medical room where she helped Martha with her daily tasks.

"We're expecting guests," Jack added. "The sort you'll want to study carefully, Martha."

"Guests who like cold baths?" Martha queried.

"Guests who like cold seawater," Jack corrected her. He reached for a sack waiting beside the pool. It contained what looked like green soap flakes, but when emptied into the pool the water gave off a smell that was nothing like Persil.

"Dried brine?" Martha queried.

"Essence of the Bristol Channel," Jack confirmed. "It was never a problem before the barrage made the Bay into a freshwater lake."

"I still don't see...." Martha began. Jack reached for his ringing mobile and confirmed that he was ready. Moments later Ianto and Alun came in through the garage entrance, each carrying dripping wet body bags. They had obviously expected a less than salubrious operation since they were dressed in jeans and waterproof jackets.

Jack took custody of the two bags and placed them in the pool of seawater before he opened them. Martha and Una both watched as two 'somethings' slithered out into the water.

"You're kidding me!" Martha exclaimed as Una gave a surprised gasp. "You are SO kidding me. MERMAIDS!"

"One mermaid, one merman, technically," Jack replied. "They will be dehydrated from their journey, but after an hour or so acclimatising, I expect you would like to do full bloods on them and any other tests you feel would enhance our knowledge of this rare species.”

“Yes, I would,” Martha replied. “Are they up for that? Are they friendly?”

“Not especially,” Jack answered. “They’re ‘co-operating’ at the moment. They don’t have any choice. It’s this or the Findus factory.”

“You’re not serious!” Martha exclaimed. “No, you really aren’t serious that time, are you? There ISN’T a Findus factory in Cardiff.”

“THEY don’t know that,” Jack answered with a wry grin. “Besides, when a pair last strayed into our purview there was a fish and chip shop up top where Beth sells postcards, now. That really had them worried.”

“They’ve been here before?”

“Why do you think it’s called Mermaid Quay up there?”

“Poetic licence,” Martha suggested. She looked again at the strange creatures. They weren’t, as folklore would have it, especially beautiful. Quite the opposite, in fact. They both had grey-green skin, the colour of the sea. The male was bald and from a distance might be mistaken for a seal. The female had straggly hair down her back that would be indistinguishable from seaweed in the water. Both had rather disagreeable expressions and small sharp teeth that could probably sever a child’s arm.

Jack brought a bucket of freshly caught cod he had purchased at the back door of a local restaurant and threw two of them into the water. The merpeople seized them and tore into the flesh with the same repulsive relish that Gollum ate his fish supper. That just about put paid to any romantic notions about their species.

“Disney has a lot to answer for,” Martha remarked before turning away and settling Una at her studies then heading to her work down in the medical room. Meanwhile Ianto and Alun returned from somewhere private where they had changed into their more familiar suit and tie daywear and went to work in the hothouse with some newly germinated alien plants. Shortly after that, Gwen arrived with Sam, who she had been babysitting while his two foster dads had been dealing with the merpeople.

Sam was immediately fascinated by the new arrivals. Gwen was still taking her coat off and firing up her computer when he wandered towards the pool, making a soft, and quite unthreatening sound in the back of his throat.

The merpeople responded in a manner that was completely threatening, rearing up and baring their teeth at him while snarling in a very unpleasant way.

“Get back, you damned animals,” Jack cried out, putting himself in front of Sam. “Get those teeth put away and swim back to the deep end. You’re here on sufferance until we can organise transport to take you back where you belong. I can keep you in a really small tank down in the sub-basement if you can’t behave properly. Or give you to the bistro for the lunchtime fish special if you REALLY piss me off.”

Ianto and Alun had come running from the hothouse, of course, but they stopped and stared at the remarkable sight of Jack defending Sam so vehemently.

The two merpeople faced off with Jack briefly before flipping backwards into the water and swimming out of his reach. He turned to Sam who was crying softly in shock.

“Go on to your comfy corner and read one of your books,” Jack said relatively gently. His feelings about the Human-Weevil hybrid were softening the more Human Sam was becoming with the plastic surgery and hormone treatments. In any case, he was a part of the Torchwood fabric by now, whereas these vicious sea beings were not.

“Back to work everyone,” he said when Sam had gone on his way. Una found him the book he was reading this week and a sippy cup of milk to soothe him. Ianto and Alun disappeared back into the hothouse where what they were doing may or may not constitute work. Jack could recall plenty of times he had taken Ianto in there for anything other than botany.

He went down to the medical room. Martha was the only one he hadn’t briefed early in the morning when he knew they were taking delivery of the unusual specimens. She had questions he knew he ought to answer.

“Yes, merpeople exist,” he said as he came down the steps to find her in the middle of testing sample slides of Gilly’s alien blood, measuring the rate by which her metabolism had been slowed since she became part of the same strange Torchwood family as Sam and Una. The alien girl who had arrived on Earth through the Rift got on with her own tasks quietly and didn’t interrupt the Human conversation.

“That much I can firmly believe at this point,” Martha replied. “Not very nice, are they?”

“They were not originally native to Earth. As far as we know, the first colony of them arrived about the time that Erik the Viking was discovering Greenland. They came from a planet which was losing its oceans as its orbit came too close to its sun. The coldest parts of the North Atlantic suited them well enough and they didn’t really cause any real problems. They occasionally showed their faces to sailors who took strange stories back to port and spawned a lot of artist’s impressions, mostly far too kind to them, as you will have noticed. Then the steam ship was invented. Things got busier - humans came into their habitat more often. Now and again a pair would follow a ship further than they intended. In the shallower waters around the Irish Sea they would get tangled in trawler nets, caught in rip tides and washed up on beaches. Some of them just got chopped to bits by propellers and eaten by the birdlife. Needless to say, Torchwood have been keeping them a secret since we started.”

“Because they’re a unique alien species?” Martha asked.

“Partly that. Partly because there would inevitably be some bloody game fishing tosser out there who would want one as a trophy. They’re nasty little bastards, but I wouldn’t like to see one stuffed and mounted on somebody’s wall.”

“And governments trying to train them to attach mines to enemy ships, I suppose,” Martha added.

“Well, yes - though it would be a case of the pot calling the kettle on our part. Torchwood had a go at that sort of thing during the war. We were a lot less kind to the waifs and strays of the universe, back then, and besides, the boss was very big into service to King and Country. He thought the merpeople ought to be ready and willing to help fight Hitler.”

“What happened?” Martha asked, despite herself.

“The little bastards just dropped the mines in the bay where they were a danger to OUR shipping,” Jack answered. “They don’t care about our Human wars. The Battle of the Atlantic was a huge nuisance to them with all the extra traffic going over their nests, but as long as no bombs actually landed on them they didn’t care.”

“I suppose I can see their point,” Martha considered.

“Well, don’t lose any sleep over their welfare. They’ve got water and fish. It’ll take a few days to organise something and then they’ll be gone.”

“I won’t,” Martha promised. “But I definitely DO want to get blood and do some DNA tests. I mean, there are questions. Are they Human at the top and fish at the bottom, or a mixture of both as a whole? Are they affected by ocean pollution? How deep can they dive? How do they reproduce?”

Jack laughed.

“Funnily enough, for me, that’s something I’ve never cared to find out. Even the women are not really appealing sexually. The men are a real turn off.”

Martha laughed with him then shooed him out of her domain. He went to make sure the merpeople were behaving then retreated to his own office where he began his negotiations with the Royal Navy to transport the not so very welcome visitors back to their proper part of planet Earth.

Their presence in the Hub was slowly forgotten as the day wore on. Everyone was a little annoyed about the way they had snarled at Sam’s perfectly harmless approach. Una was especially disenchanted with them and ignored them even when they splashed noisily. Ianto and Alun also held their attitude towards Sam against them. Gwen thought they were ugly and moody and was too busy with her own project to care.

At the usual time, everyone prepared to go home for the evening. Jack was the last out. He made sure the merpeople had enough food and reminded them again that there were several fish restaurants in the bay area then he left them to their own devices. They couldn’t get around particularly well out of the water. The fantasies about them growing legs when they dried off certainly belonged with Disney. They couldn’t cause any trouble overnight.

Just in case, he left the security cameras running and switched on an invisible shield around the pool that would deal a nasty electrical shock to anyone trying to overstep their boundaries.

Then he headed home to his family, something he had only rarely been able to do in his long association with Torchwood, and which he wasn’t going to sacrifice to be probation officer to a pair of stroppy fish.

A little after two in the morning, an alarm woke Jack. He leapt out of bed abruptly and tripped over his clothes scattered on the floor. He swore loudly and reached for the bedside lamp. Garrett beat him to it.

“I was trying not to wake you,” Jack complained as he grabbed his underwear and started dressing.

“Fat chance. That was the remote alarm for the Hub. Torchwood is being burgled?”

“I don’t know what’s happening,” Jack answered. “But I have to get down there.”

He finished pulling on his clothes, then checked his gun and slipped it into a shoulder holster.

“Do you need me?” Garrett asked. “For the situation at the Hub, I mean, not cuddling.”

“The boys will have got the alarm at the same time. They’ll be heading to the Bay as well. We’re covered. You try to get back to sleep.”

“I should be so lucky. Let me know you’re ok once the situation is secured.”

“I will,” Jack promised. He spared a moment to kiss his lover then he headed downstairs. Garrett listened to the sound of the SUV roaring down the road then got up and made coffee for himself. He wasn’t going to sleep again tonight. Whatever Jack said, an intrusion at the Hub was serious, and however cool he tried to be about it, he would definitely worry until he knew Jack was safe.

That was the price of family for men in their kind of work.

Ianto and Alun got there just before Jack. Their apartment was closer to the city centre and on a more direct route to Mermaid Quay. Even so, they were too late to stop the disturbance within the most secret installation in the British Isles.

"How the fuck did they do it?" Jack demanded. “How did the bastards get in here?”

"Nobody DID, sir, " Ianto answered. He was at his own workstation carefully studying the CCTV footage from within the Hub. "I mean, nobody broke IN here. It was the merpeople. They broke OUT, after overriding every protocol and infiltrating every level of the Hub.”

“How? They don’t have any fucking legs! And what about the bloody electric fence I put around the pool?”

Ianto pushed a key and displayed the CCTV on the whitewashed side wall. Jack watched in astonishment as the two merpeople leapt out of the water and through his electronic shield as if it wasn’t there. He saw blue arcs around their bodies, but they were completely unaffected.

“It’s electricity. They’re WET. They should have been pan-fried,” Jack complained.

“They seem to be able to shield themselves,” Ianto answered him. “Sorry.”

Jack continued to watch the two merpeople move in a series of flops, wiggles and leaps that covered the length of the Hub in a very short time. They managed stairs in the same way, moving down to the deep archives where alien artefacts were kept. They went through the archive carefully, clearly looking for something.

And they found it. Jack and Ianto both watched as the merpeople opened one of the steel cabinets with individual coded electronic locks.

“HOW did they do that?” Jack asked.

“I think….” Ianto began slowly. “If I had to guess… I would say that they stored the electricity from your ‘fence’ within their bodies somehow and used it to override the lock, thus opening the cabinet.”

Jack’s expression was black. Ianto was glad none of this was his fault. He didn’t want to be on the wrong side of him right now.

“Is it really that easy to break through all our precautions?” he asked. “Are we really so vulnerable?”

“I didn’t think so,” Ianto answered him. “But it has to be said that our security is mostly invested in our outer defences. The merpeople were already inside. That was our Achilles Heel in this case.”

“Fuck!” Jack responded.

“You’ve used that word more often than Owen Harper tonight,” Ianto told him gently.

Jack said something else, possibly in a language not known on Earth, and shook his head. Ianto understood fully. Jack was feeling personally violated, humiliated, by the way these apparently powerless creatures had deceived him and then gone on to rob him.

Rob HIM. That’s how it felt to Jack. Torchwood was his organisation. He had worked for it since its inception in the Victorian era. He had inherited the directorship at the turn of the twenty-first century and had recruited every member of the team since. It was HIS Torchwood that they had rampaged through and stolen from.

“Jack….” Ianto began. But what could he say? It was almost like trying to console a rape victim. At best it was like facing somebody whose house had been burgled. He was feeling it that deeply.

“What did they take?” he asked. “What did they come here for?”

That much was obvious, by now. It was no mistake that had brought them into the estuary to be accidentally picked up by a trawler whose crew had been subjected to a large dose of Retcon. They had come with the full intention of being brought into Torchwood, in order to find something they wanted.

Ianto switched the CCTV from recorded to live feed. Alun was there, now, examining the scene of the crime, the opened cabinet. He confirmed that it was empty, and noted the reference number that would tell them what was missing.

“Cariad?” Ianto queried as Alun spoke to him on the in-ear com. “Ok, thanks. You might as well come on back up. There’s obviously nothing else to see.”

Ianto turned back to his workstation and typed in the fifteen character reference Alun had told him. The missing artefact was displayed on the wall along with details of its function.

“It came into Torchwood’s possession in 1969,” Ianto explained. “It was called The Philosopher’s Stone because it is a device which alters elements. It can literally turn lead to gold, or back again, a bit of quartz into a diamond, or transform the elements that make up water into oil… or anything you can ask for. I think it was archived because it was too tempting or something.”

“It was shelved because it was too dangerous,” Jack told him. “Lead to gold… There wouldn’t have been a church in Cardiff with a roof left on if that got into the wrong hands. And that’s assuming greed was the only motive. Everything is made of elements. The Human body is made up of elements. Suppose somebody decided to turn their wife into salt!”

“What do THEY want with a gadget of that sort?” Alun asked as he slipped back into the Hub and overheard the conversation.

“I doubt they want to turn anything into gold,” Jack replied. “But I am not downplaying this situation in any way. Whatever they’re up to, it can’t be good for the Human race. We have to stop them.”

“But how?” Alun asked. “They got away through the waterside entrance. They’ll be well outside the bay by now. The chances of catching them even with a whaling ship and harpoon….”

“How about if they had transponder signals we can follow?” asked Martha. Jack spun around in surprise as she stepped towards him. “The boys called me. They needed somebody to look after Sam. I brought Una and Gilly to their place. They’re both sensible enough to manage while I lend a hand, here.”

“What’s all this about transponder signals?” Jack asked.

“When I was doing the blood tests I inserted microchips in both of the merpeople. I had this idea about tracing their migratory patterns or something. I know I should have asked you, first, but it was a spur of the moment idea.”

“I’ll tell you off about it later,” Jack said. “Meanwhile, let’s go get those fish-tailed bastards before they get too far away.”

“How?” Alun asked.

“Oh, fantastic!” Ianto grinned. “It’s ages since we used that.”

“Used WHAT?” Martha asked.

“Never you mind, just go get those transponder codes and follow us down the bay entrance.”

Martha was puzzled. She had used the ultra-secret Torchwood tunnel that led to a small subterranean marina occasionally. A swift motor launch was kept there. But surely it couldn’t be used to catch up with the merpeople?

The motor launch was there, sleek black with Torchwood stencilled on the side just like the SUV. But Jack didn’t go near it. Instead he tapped keys on his vortex manipulator and a section of wall opened up. Something gun metal grey slid forward on runners into the water.

“A submarine?” Martha queried.

“I didn’t know we had a submarine,” Alun added, equally surprised.

“It doesn’t get used very often,” Jack told them. “It uses way too much fuel.”

“It’s a Holland prototype,” Ianto explained proudly as Jack opened the airlock into the three man cockpit.

“And that means….” Martha asked.

“Phillip Holland, an Irishman who emigrated to America invented the first working sub in the 1880s,” Ianto continued. “This is one of the prototypes. Torchwood got hold of it when it was new. Jack has souped it up a bit over the years. It has a modern engine and some extra safety measures like a proper airlock, not just an airtight door, and all the state of the art sonar equipment, but it is basically a Victorian submarine.”

“Room for one captain, one co-pilot, a navigator and – originally – a gunner. We don’t need a gunner, but you two can take it in turns to monitor those damn fish.”

He was talking to Alun and Martha, who both realised that they were being invited along as passengers. Ianto slid into the cockpit after Jack and buckled himself into the co-pilot seat next to his boss and former lover.

“Jack taught me, ages ago,” he explained. “We used to take it out past the Barrier and into the channel – even out into the Irish Sea a couple of times.”

“Even though it takes so much fuel it isn’t used for missions?” Alun queried. Martha raised her eyebrows, too. This had been their idea of a romantic drive? The submarine was a good twenty feet long, but a lot of that was engine. The cockpit was pretty cramped and the lighting was dim. There was a curving windscreen at the front and round portholes at the side through which the murky underwater world of Cardiff Bay was revealed in the gloom penetrating halogen lights, but that did little to take away the claustrophobic feeling.

Martha fed the transponder code into the very sophisticated scanner that was very definitely not made in the Victorian era by an ambitious Irishman. There was no response. The merpeople were already out of range. She was resigned to this being a waste of time.

“I think I know what they stole the Philosopher’s Stone for,” Alun said after a long silence broken only by the hum of the ‘souped up’ submarine engines. He had been looking, ever since they set off, at the gauges that measured how much air there was in the cockpit. He had been subconsciously thinking about air and its constituent elements.

“Do you intend to share?” Ianto asked him after another half minute passed.

“The gismo can change elements into other elements. What if some of the oxygen in the air was changed into hydrogen - in the proportions two parts hydrogen to one part oxygen?”

“Oh shit!” Jack exclaimed.

“I don’t get it,” Martha started to say, then she, too, worked it out. “If you mix two parts hydrogen to one part oxygen in a test tube you create water. What if it was done in the atmosphere of a planet?”

“It would turn the air to water,” Ianto replied.

“But could that small machine DO that?” Martha asked.

“It could be the catalyst to start the change,” Jack told her. “Alun is right. That’s what they want. They intend to turn Earth into their own world covered in water – their idea of paradise.”

“Oh, God!” Martha expressed the horror everyone felt as the full consequences of that action sunk in – absolutely no pun intended. The sea levels would rise. The land would be swamped – except, perhaps, a few very high mountain peaks. The sentient creatures of the sea would rule a changed planet with no room on it for the Human race.

“This is about more than wanting to kick scaly backsides over what they did to us,” Ianto whispered to Jack. He nodded grimly. He knew that all too well.

“I think I’ve got a signal,” Martha called out. “Yes, it’s the transponders – both of them. They’re together.”

“Give me a bearing,” Jack told her.

“How?” Martha asked. “I get how to recognise the transponder signals, but bearings… If I wanted to be a sailor I wouldn’t have taken a medical degree.”

Alun knew how. He read out the bearing to Jack who piloted the submarine skilfully.

“Dare we enquire how you know how to do that?” Martha asked him.

“It’s not much different from the little Chula space ship I had when I first met our mutual friend, The Doctor. Besides, I got to know a few sailors during the war.”

“Too much information, thank you,” Alun told him. “Though it is nice to know that ‘Captain’ isn’t just an honorary title. You’re doing fine as our U-Boat Captain.”

Jack laughed, though at a joke that only made sense to him and would take far too long to explain.

“Where are we, exactly?” Martha asked. “I mean… in relation to the dry part of the world?”

“We’re off the south coast of Ireland,” Ianto told her. “I could give you exact longitude and latitude if you like, but it wouldn’t mean a lot to you.”

“Ireland is close enough,” Martha admitted. “Are we going to have to follow them right out into the Atlantic? Do we have the fuel for that? Jack, you said about this thing using a lot….”

“They’re not going to the Atlantic,” Jack answered. “That signal is stationary. They’ve got to where they’re going, and I realise now just what a thoroughly nasty bunch they are. To choose a place like that for their nest….”

“A place like what?” Alun asked.

“The wreck of the Lusitania,” Ianto answered as a ghostly shape came into view ahead of them. As they neared it they could all recognise the shape of a huge ship that had been broken almost in two just forward of the bridge, exposing the decks to the ocean. A mast was still surprisingly intact though covered in fronds of seaweed. As they drew closer they saw one of the merpeople swim down the length of it towards the bulk of the ship.

“It was a passenger liner crossing from New York in 1915,” Ianto continued for those not fully conversant with maritime history. “It was only a few miles from safe harbour in Queenstown, in County Cork, when it was torpedoed by a German sub. Thousands died. A lot of the bodies were never recovered.”

“It went down in minutes,” Jack added. “People didn’t have a chance. Trapped in the dark, the water rising….”

There was a question on everybody’s lips but nobody wanted to ask it. Jack saved them the trouble.

“One of the very worst places to die – in an elevator in a sinking ship,” he said. “Ianto, take over from me. Keep her slow and steady over the wreck.”

Ianto seemed to be the only one who had realised what he was doing. Martha turned her face away as he started to strip his clothes. He actually stopped at a t-shirt and shorts so there was nothing for her to be embarrassed about, but it was an odd thing to do in such a confined space.

It started to make sense when he opened a compartment and took out a streamlined harpoon gun and a miniature oxygen tank of the sort beloved by James Bond. With those tools he headed for the airlock.

“He’s going after them?” Alun and Martha were both surprised by his actions. Again, Ianto seemed less disturbed. He did as he had been ordered, keeping the submarine moving slowly around the tragic century old wreck.

“They hurt him. He wants revenge,” he said quietly.

“As well as getting the artefact back,” Alun reminded him. “And stopping Earth’s oxygen from being transformed to water.”

“Yes,” Ianto agreed. “Yes, those things are important, too. But getting even with them is first in his mind. Don’t blame him for that. Don’t hold him up to some moral standard he can’t achieve. He’s Jack. He looks and acts like nothing touches him, but underneath he’s all about his emotions – and they’re driving him, right now.”

Nobody disagreed. Apart from Martha, who had been to the end of the universe with the Captain, – a story she had never fully shared with anyone else – Ianto had been with Jack the longest. If anybody knew anything about the immortal mystery man with a thousand tall stories – and that was held in doubt – it was him.

What form his revenge was going to take wasn’t quite apparent, though, until the body of a mermaid crashed against the windscreen, a harpoon bolt through its breast, deep red blood turned an even deeper colour in the night vision. The face, never a picture to begin with, was twisted in agony and possibly anger at the nature of her death.

The phrase ‘one down…’ went through the minds of all the Torchwood team. They fully expected to see another body float into view. They were not expecting four bodies, one broken in half at the point where ‘human’ became ‘fish’. While they were debating how even a thoroughly enraged Jack could have managed to do that, they saw something even more peculiar in the front view. The wreck of the Lusitania took on a strange, silvery glow as a kind of bubble squeezed out through the huge crack in the hull and enveloped it.

The bubble remained for several minutes before dispersing almost as quickly as it appeared. When the view was clear the water around the wreck was littered with the rapidly disintegrating bodies of dead merpeople. Through the miasma Jack swam towards the submarine, clutching the Philosopher’s Stone. He was struggling, though, something clearly sapping at his strength. He almost dropped the dangerous artefact.

“Is there another of those air bottles in there?” Alun asked, pulling off his clothes all the way down to his underwear.

“You’re going out?” Ianto queried. “I’m more experienced at scuba diving than you.”

“You’re also more experienced at piloting a sub. I know sod all about that. I did a bit of diving in the army. I can manage. Martha, it looks like he might need your talents when I get him back in.”

Martha nodded and found the first aid kit that was essential equipment aboard any sea-going craft while Alun slipped through the airlock and appeared in the front view swimming towards the floundering Jack. She was relieved to see it was an up to date one, not from the launch of the submarine itself. She was making space to treat a patient when Alun re-opened the airlock and dropped a half-conscious Jack down first.

“This looks like mercury poisoning,” Martha exclaimed in surprise as she made a quick but thorough examination. “He shouldn’t have ingested any since he was using an air tank, but his skin and eyes are contaminated. Alun, don’t worry. I’ll deal with him. You get your wet clothes off and wrap yourself in a thermal blanket.”

Jack was still fighting to stay awake. He squirmed as Martha pulled off his vest and shorts. She slapped him gently.

“Behave. Don’t tell me this isn’t one of your wildest dreams – being stripped off by a woman in a confined space.”

Jack said something crude that earned him another slap, then co-operated fully with her efforts to treat him. Dying of mercury poisoning wasn’t pleasant. He was glad not to have to this time.

The only thing available to wash his skin was a bottle of saline solution, but that did the job. Martha gently washed his eyes and ensured there was no damage before cleaning the rest of his skin and wrapping him in an emergency blanket. Ianto, meanwhile was piloting the submarine back towards Cardiff. The mission was clearly accomplished and it was time to get home.

When they reached the Hub Martha insisted on putting Jack to bed in the medical room for what was left of the night. He woke mid-morning and she let him get dressed, after passing him fit.

“You used the stolen gismo to turn the water around the wreck into mercury,” she said. “Killing the merpeople, of course. They couldn’t possibly breathe in it. But I presume you didn’t leave that much mercury in the water. The impact on fishing in Irish waters….”

“I turned the mercury into fresh water,” he confirmed. “They can’t live for long in that, either. Any survivors should sod off back to the north Atlantic. But I think I probably wiped out the whole colony.”

“Do you feel good about that?” Martha asked him.

“Not especially. But they messed with us, first. I did what was necessary. I’m not going to lose sleep over it.”

“It’s your conscience. You resolve it however you want.”

“I always do,” Jack answered darkly. For a moment Martha wondered just how many more massacres of that sort he had instigated. Then she decided she didn’t need nor want to know.

“Go and see what the boys have done with your mermaid pool,” she said. “It’ll put a smile on your face.”

Jack didn’t know what she meant, but as he emerged onto the balcony at the top of the medical room stairs he was aware of gleeful laughter from child voices. He looked down to see Una and Gilly both skating gracefully on the frozen water while Sam lumbered about, slipping and sliding and occasionally falling down, but enjoying himself all the same.

“One of the other archived objects,” Ianto told him. “A stone that turns fresh or salt water to ice at room temperature. Sam couldn’t possibly go on the ice rink they put up on the Plas for Christmas. The girls are loving it, too. Gwen looks like she’s itching for a go once she’s finished her report on dwarf alien shoplifters in St Mary’s Street.”

“It can stay until after New Year,” Jack conceded. “As long as everyone gets their work done before they play. That goes for the kids as well as the grown-ups… assuming there is any difference around here sometimes.”