Jamie disabled the last of the coach engines and turned to look down the barrel of a rifle aimed at him by a soldier who hardly looked much older than he was.

“Stop right there, Sonny Jim,” the soldier commanded him in a tone that tried to sound as if he WAS much older and with more authority than a mere civilian.

“I wasnae doing nae wrong,” he protested, holding out his hands to show that he wasn’t carrying a weapon or any form of contraband.

“Get your hands up and don’t move. We were told to look out for looters. You’re under arrest.”

“I’m nae looter,” Jamie insisted. “I was just doing what The Doctor told me to do.”

“The doctor?” The soldier looked less certain of himself. “You mean THE Doctor?”

“I dinnae know about THE but he’s the one I answer to,” Jamie responded. “He wanted these mechanical beasties switching off so I did as he asked.”

“Just don’t move for a minute while I check this out.”

The soldier kept his rifle trained on him while he pulled out his radio and called his command centre. Jamie overheard a pair of call signs he had heard before followed by a question that made him smile.

“Greyhound Three to Trap One Mobile, I have a situation here in the coach station.”

“Trap One Mobile to Greyhound Three, is there a blue box where you are?”

“A blue box... you mean THAT blue box? Trap One Mobile… do you mean the….”

“It’s all right, Private Norris,” said a confident sounding corporal who approached with The Doctor at his side. “These aren't looters. The lieutenant is coming in with the third member of the party.”

Jamie looked around to see a Land Rover turning into the coach bay. Zoe jumped out of the passenger seat as soon as it stopped and ran to hug The Doctor and Jamie. The lieutenant got out of the driver’s side and walked around to greet them.

“Doctor, this is a turn up for the books,” said the cheerful East Lancashire tones of the man they last met as Sergeant Benton. He was now wearing the insignia of a second lieutenant on his uniform.

“I found him,” Zoë announced proudly. “I found him and UNIT.”

“It’s good to see you, Mr Benton,” The Doctor said.

“Good to see you, Doctor, especially YOU, you.”

Jamie and Zoë both looked puzzled about that remark. The Doctor was less concerned with that issue.

“Is the brigadier still in charge of you all?” he asked. “Brigadier Alistair Gordon Lethbridge Stewart.”

“He most certainly is,” Benton answered him. “He told me to bring you and the TARDIS to the mobile command post. If you’d all like to hop into the Land Rover I’ll leave the transport of the old girl to the lads here.”

The Corporal and Private immediately went to do as they were told, calling on two more of their number to help tip the TARDIS and manhandle it out of the coach station. Zoë hoped they would be able to call upon some sort of transport rather than lugging it all the way to wherever U.N.I.T.’s mobile headquarters was.

She resumed her place in the passenger seat of the Land Rover while The Doctor and Jamie climbed into the back. Benton reversed the vehicle out of the coach park and drove the way Zoë had walked earlier. She noticed that an army ‘Green Goddess’ fire truck had arrived and soldiers were dealing with the chip shop fire.

“Thank goodness for that,” she said out loud.

“It wasn’t easy,” Benton replied. “Even persuading them that there was a fire to tackle was a problem. This whole zone is troublesome. The Brigadier will explain, of course. But I’m wondering…. Of course, the TARDIS can move in space and time. Even so… seeing you again like this, Doctor… this version of you... We’d all got used to the other ones.”

“Other ones?” Jamie queried. “There’s only one Doctor.”

“Indeed, there is,” The Doctor confirmed. “But I think Mr Benton is trying to say that future versions of me have worked with U.N.I.T. since we last saw them in the 1960s. That, of course, is perfectly possible. But those events have not happened for me, yet. It is important that nobody tries to talk to me about them. It would be a terrible paradox, and I’m not sure London as it is just now could take such a thing.”

“Do you think we might BREAK it?” Benton queried.

“We very well might,” The Doctor replied. “Paradoxes fracture causality. Too many of them and serious problems emerge – the sort that even Humans scurrying about at their ordinary lives and ordinary problems might start to notice. That’s not good. Besides, it really isn’t a good idea for me to know too much about my own future.”

“Well, just don’t let The Brigadier start reminiscing about old times, then,” Benton warned him. “Remind him there IS an emergency going on.”

“It is a relief to find U.N.I.T. aware of the emergency,” The Doctor confirmed. “I shall certainly keep The Brigadier on the subject. I just hope what he calls military intelligence has something useful to report.”

Benton ignored th at slur upon Human endeavour against alien interference as just The Doctor’s usual way.

“I think The Brigadier might be hoping YOU can help him out a bit,” he replied.

The Doctor sighed.

“HE did say, this morning, that it was a pity you weren’t around, because this kind of mystery would be right up your street, something you could get your teeth into. And I’m SURE he meant it as a compliment.”

The Doctor smiled just a little smugly.

The Land Rover drove on beyond the place where Zoë had found Benton and his men until they reached the Thames at the place where Grosvenor Road was intersected by the Chelsea Bridge and the wide viaduct that carried several lines of railway across the river. This ought to have been a busy place for traffic and pedestrians. The emptiness was eerie and disturbing.

Benton stopped the vehicle outside an elegant Georgian building with a pair of lamps shaped like glass eggs either side of the entrance steps. The brass plaque by the door and a row of flags fluttering in the slight breeze declared that it was a Royal British Legion Club. The presence of soldiers of various ranks and an assortment of military vehicles parked outside seemed perfectly congruous in such a place.

The Brigadier had set up his command post in the first floor Members Bar. The radio operator occupied the area beneath the dartboard and was busily communicating with Traps 2 and 3. A huge map of London partially covered the British Legion standard on another wall.

The Brigadier was sitting at a table with a glass of single malt from the bar and a pile of reports and memos hastily written on official U.N.I.T. headed paper. He looked up to see The Doctor and his two companions and opened his mouth in surprise before deciding to accept the situation.

“It’s good to see you, Doctor – any one of you, especially at this uncertain time. I was hoping that you would know, somehow. I really expected… but any way, you’re here, no matter which one of you it is.”

That sentence would have been absurd in any other circumstances and The Brigadier was sure it wasn’t his best greeting, but the worst was over now. He sent one of his subordinates to bring tea for Zoë and Jamie. He offered The Doctor something ‘stiffer’ but he declined, preferring the tea.

“What’s happening, here, Brigadier?” The Doctor asked while the tea was being poured. “Why the evacuation?”

“We didn’t evacuate,” The Brigadier answered. “The people just disappeared. This is the fifth time it’s happened in as many days. Each time the ‘outage’ is as much as three hours before everyone is returned to exactly where they were before. We’ve interviewed dozens of them, and they seem to have had their memories changed – modified – to make them think there was nothing unusual happening.”

“It’ll be tough doing that with all the people missing from the station,” Zoë pointed out. “Their coaches will all be three hours late. They’ll be asking questions.”

“They’ll be asking questions about why they’ve been waiting for three hours,” The Brigadier explained. “That’s how it worked.”

“And why doesn’t anyone know that they’re missing?”

“That’s the other thing,” The Brigadier continued. “It’s as if everybody outside the affected area forgot for the duration of the ‘outage’ just forgets that the area exists. Trains that ought to have terminated at Victoria Station all stopped at the last station outside the area. Traffic just goes around it. Yesterday was very strange. The affected area includes Buckingham Palace. Everybody forgot that we had a royal family.”

“How is that possible?” Jamie asked. “People cannae just have their minds changed. They cannae just forget that there’s a king…..”

“A queen, actually,” The Doctor corrected him. “How is it that you and your men weren’t affected?”

“We were to begin with, but one of our science bods drew our attention to the problem. He came up with a way for us to see the invisible area. It was actually an adaptation of the earpieces you designed to stop us being affected by the Cybermen.”

The Brigadier showed The Doctor one of the miniature gadgets. He looked at it very carefully and nodded in approval.

“A very effective gadget. I’m impressed by your ‘bods’. That would do the trick nicely.”

“I don’t understand it myself,” The Brigadier admitted.

“It blocks perception filters,” The Doctor explained. “What you have here is an incredibly wide range filter causing mind disruption on such a massive scale. It ought to be impossible. If I hadn’t seen it myself….”

“Why impossible?”

“It would take a huge amount of energy… the output of a whole electricity power station. Even then, it would only last a very short time – only about… well, three hours, give or take.”

“So there must be a power source,” Zoë concluded before anyone else could say it. “Have you looked for it?”

“Since the idea had not even occurred to us, no,” The Brigadier replied.

Zoë stepped towards the map and studied it carefully for a few minutes.

“It would be so much easier with computer generated charts,” she sighed. “But even on this paper map some things are obvious. I’m really surprised you hadn’t seen it before, Brigadier.”

“Seen what?” The Brigadier stood and went to see what she was looking at. “I really don’t understand.”

“Look at the pattern, here. All the ‘outages’ over the past five days. You’ve marked it out here, the way it sorts of fans out, the first section to go – then the next, over a total of seven hours, but each section being affected for three. But it’s like a triangle getting wider and wider until it forms a semi-circle… with its centre of origin on this spot.”

“Which spot?” The Doctor asked, coming to look at what Zoë had worked out. “Oh, yes. Very interesting.”

“What?” The Brigadier was puzzled. “What are you looking at? I don’t see it.”

“That!” Zoë’s slender, feminine index finger jabbed at the centre of origin rustling the crisp map paper. The Doctor saw exactly what she meant. The Brigadier still looked blank.

“Ach!” Jamie exclaimed impatiently. He stalked to the Georgian window set above the line of flagpoles. He looked across the trees that lined the Thames at this point in its journey through London to the unmistakeable landmark on the other side. “What dae ye call THAT, Brigadier?”

The Brigadier came to the window, but again he was unable to see what he was looking at. It was as if the four slender towers of the iconic 1920s Art Deco building wasn’t there in his vision.

“Brigadier, give me your ear piece,” The Doctor commanded. The Brigadier did as he said straight away. The Doctor brought from his pocket the device he called his sonic screwdriver though it was rarely used for that purpose. He used it now to create an ear-splitting noise from the miniature perception filter blocker. When he was done he gave it back to The Brigadier who put it back into his ear dubiously.

“Good gracious!” he exclaimed when he looked again. “Battersea Power Station. How could I have missed something as obvious as that?”

“Because there is a SECONDARY perception filter covering the Power Station. It must be working constantly so that, effectively, that patch of London has vanished from the consciousness of Londoners. It must be playing havoc with the Underground system. But more importantly, I believe we have found the power source, and almost certainly the core of this mystery.