My decision to visit Lange this morning was a spontaneous one, so my follower must have been with me since I left Tante Lisl's before breakfast, and that meant he'd probably been outside the hotel all night. All night on Christmas Eve; where do you find such dedication these days? From Tante Lisl's he must have used a car, otherwise I would have spotted him earlier. He'd have found it easy enough to anticipate the speed and direction of a solitary walker in the almost empty streets. I should have noticed the car right from the start. I was becoming too old and too careless. He stopped again; he must have guessed he'd been spotted, but he was still sticking to the book, ducking out of sight and keeping his distance. He was inexpert but diligent. It was easy to guess that he'd hoped to do the whole job from inside a car, hence the brightly coloured car coat, but now that I'd come poking about in the Tiergarten, he'd had to get out of the car and earn his money. Now he was conspicuous, especially with that big parcel under his arm.

I looked back. I couldn't see his car but he hadn't had many alternatives about where to leave it. I walked west, uncertainly changing direction but heading southward enough to keep him hoping that I would return to where he'd left the car. Was he alone? I wondered. Surely no professional would try to tail a suspect without any assistance whatsoever. But it was Christmas and perhaps all he had to do was to report my movements. He wasn't a private eye; whatever their shortcomings, they can all follow an errant husband and stay out of sight. And if he wasn't a KGB man and he wasn't a private eye, what was left? One of our own people from the Berlin Field Unit? Even my advanced paranoia couldn't believe that one of those lazy bastards could be persuaded into action on Christmas Day. Now I strolled back towards the park. I stopped to examine the trunk of a tree where someone had carved a hammer and sickle that was bent to become a swastika. I used the chance to watch him out of the corner of my eye. The parcel slipped from his grasp and he took his time about picking it up. He was right-handed; well, that was a useful thing to bear in mind.

I paused again at the little river in the park. But today the famous Berliner Luft was too cold for water to survive in. There were two people skating on the ice. A man and a woman, elderly judging by their stately posture and the way they skated side by side, long overcoats, flowing scarfs, and heads held high, like an illustration from some nineteenth-century magazine.

I hurried along the path as if suddenly remembering an appointment. Then I stooped down to hide. It wouldn't have worked with anyone more experienced, so it was really a test of his expertise. I still had no measure of him and couldn't guess what his motives might be. As it was, he walked right into it. That is to say, he walked right into me. It was the hurrying that did it; it often stampedes the pursuer into incautious and impulsive actions. That was how Hannibal won the Battle of Lake Trasimene after crossing the Apennines. All it needed was that sudden dash towards Rome to make Flaminius chase after him and blunder right into his ambush. Hannibal would probably have had the makings of a good field agent.

'Don't move,' I said. I had him from behind, my arm round his throat and the other twisting hell out of his right arm while he was still looking for me far down the path. He grunted. I was holding his neck too tight. 'I'm going to release you,' I said, 'but if you move carelessly after that, I'll have to really hurt you. You understand, don't you?'

He still didn't answer properly so I relaxed my hold on his throat a bit more to let him breathe. When I let him go he bent double and I thought he was going to collapse on me. I looked at him with surprise. The arm seam of his coat was torn and his hat was knocked off. He was making terrible noises. I suppose I'd grabbed him too tightly; I was out of practice. But he shouldn't have been gasping; a young man like him, well under thirty, should have been in better physical shape. Still bent over he clutched his middle, taking very deep breaths.

'Who the hell are you?' I said.

'We'll ask the questions, Mr Samson!' There was another of them, a slim bespectacled man in a flashy brown-suede overcoat with fur collar. He was holding a gun and not bothering too much about who saw it. 'Hands behind your back, Samson. You know how these things are done.' I cursed my stupid overconfidence. I should have guessed that such clumsiness as the bearded man displayed was all part of the trick. They'd now made me play Flaminius to their Hannibal.

The bearded one – still gasping for breath – rubbed me down quickly and thoroughly and said, 'He has nothing.'

'No gun, Samson? This is not the expert we've heard so much about. You're getting old and careless.'

I didn't answer. He was right. I'd chosen not to go to Lange with a gun under my arm because it would have made it harder to deny my connection with London Central.

'Here he comes,' said the man. 'It took him long enough, didn't it.' He was watching a dented panel truck trundling over the brown grass. The skaters were nowhere to be seen now: they were all part of the same team sent to get me.

The rear doors of the van opened to reveal a gleaming wheel chair. They pushed me up onto the chair and strapped my ankles and neck to the steel framework. Then they blindfolded me as the van drove away. It was all over in five minutes.

The roads were empty. The journey took no more than twenty minutes. The blindfold was good enough to prevent me seeing where I was, but I was bumped up steps and the gates of an elevator were carelessly slammed against my arm.

They unstrapped me and locked me in a room. I was left to remove my own blindfold, not so easy when one's arms are cuffed behind one's back. It was impossible not to admire their efficiency and to deplore my own unpreparedness. There was no doubt where they'd brought me: I was in East Berlin, just a few minutes' walk from Checkpoint Charlie. But from this side of the Wall, it's a long walk back.

There were two windows. It was an anteroom – really a place where people waited. But the people who waited here had to have bars on the windows and heavy locks on the doors, and the window glass was frosted to make it difficult to see out. At the top of each window there was a small ventilation panel. I could reach that far only by putting a stool on the tabletop. With hands cuffed behind me I almost toppled as I scrambled up. Now through the narrow gap – the panel opened only as far as the bars permitted – I could see across the city. There was no movement: no cars, no trucks, no people. I recognized the massive USSR Embassy in the Linden from the shape of the roof. Nearby there was the last remaining section of the Adlon Hotel; a few cramped rooms in the rear that in the thirties were used only for the personal servants of the hotel's clients. And there were the parking lot and the hillock that marked the site of the Führerbunker where Hitler had fought his last battles against marriage and the Red Army and, defeated by both Venus and Mars, blew out his troubled brains. Now I knew where I was: this was Hermann Goring's old Air Ministry, one of the few examples of Nazi architecture to escape both Anglo-American bombers and Soviet planners.

I went back to the hard wooden chair and sat down. It was Christmas Day – not a festival that any sincere Communist cares to celebrate, but there were enough insincere ones to empty the building. It was silent except for the occasional, distant sounds of a slammed door or the hum of the lift. I looked round the room: no books or papers, the only printed item a brightly coloured poster that was a part of the Kremlin's contribution to the anti-nuke debate. But the missile to be banned was labelled 'NATO'. There was no mention of Russian missiles – just a handsome young Communist and a snarling GI. There was a second door in the room. It had a glass panel over which had been stuck patterned translucent paper. Such paper was commonly used in the East Bloc where frosted glass was sometimes in short supply. Standing with my back to the door I was able to peel a little of it back from the corner. A sticky compound remained on the glass, but I scratched it away with my fingernail.