Sitting across the table from him, legs crossed in a languid posture, there was a fresh-faced young man named Larry Bower, a Cambridge graduate. His hair was fair and wavy, and he wore it long in a style that I'd heard described as Byronic, although the only picture of Byron that I could call to mind showed him with short back and sides. In contrast to the coarse ill-fitting clothes of Valeri, Bower was wearing a well-tailored fawn Saxony check suit, soft yellow cotton shirt, Wykehamist tie and yellow pullover. They were speaking German, in which Larry was fluent, as might be expected of a man with a German wife and a Rhineland beer baron grandfather named Bauer. In an armchair in the corner a grey-haired clerk bent over her notebook.

Bower raised his eyes to me as I came in. His face hardly changed but I knew him well enough to recognize a fleeting look that expressed his weariness and exasperation. I sat down in one of the soft armchairs from which I could see both men. 'Now once again,' said Bower, 'this new Moscow liaison man.' As if reflecting on their conversation he swung round in his chair to look out of the window.

'Not new,' said Valeri. 'He's been there years.'

'Oh, how many years?' said Bower in a bored voice, still looking out of the window.

'I told you,' said Valeri. 'Four years.'

Bower leaned forward to touch the radiator as if checking to see if it was warm. 'Four years.'

'About four years,' he replied defensively.

It was all part of the game: Bower's studied apathy and his getting facts wrong to see if the interviewee changed or misremembered his story. Valeri knew that, and he did not enjoy the mistrust that such routines implied. None of us did. 'Would you show me again?' Bower asked, pushing a battered cardboard box across the table.

Valeri opened the box and searched through a lot of dog-eared postcard-sized photographs. He took his time in doing it and I knew he was relaxing for a moment. Even for a man like this – one of our own people as far as we knew – the prolonged ordeal of questioning could tighten the strings of the mind until they snapped.

He got to the end of the first batch of photos and started on the second pile. Take your time,' said Bower as if he didn't know what a welcome respite it was.

Until four years before, such identity photos had been pasted into large leather-bound ledgers. But then the KGB spread alarm and confusion in our ranks by instructing three of their doubles to select the same picture, in the same position on the same page, to identify a man named Peter Underlet as a spy, a KGB colonel. In fact Underlet's photo was one of a number that had been included only as a control. Poor Underlet. His photo should never have been used for such purposes. He was a CIA case officer, and since case officers have always been the most desirable targets for both sides, Underlet was turned inside out. Even after the KGB's trick was confirmed, Underlet never got his senior position back: he was posted to some lousy job in Jakarta. That had all happened at the time my wife Fiona went to work for the other side. If it was a way of deflecting the CIA's fury and contempt, it worked. I suppose that diversion suited us as much as it did the KGB. At the time I'd wondered if it was Fiona's idea: we both knew Peter Underlet and his wife. Fiona seemed to like them.

'This one,' said Valeri, selecting a photo and placing it carefully on the table apart from the others. I stood up so that I could see it better.

'So that's him,' said Bower, feigning interest, as if they'd not been through it all before. He picked up the photo and studied it. Then he passed it to me. 'Handsome brute, eh? Know him by any chance?'

I looked at it. I knew the man well. He called himself Erich Stinnes. He was a senior KGB man in East Berlin. It was said that he was the liaison man between the Moscow and the East German security service. It must have been a recent photo, for he'd grown fatter since the last time I'd seen him. But he still hadn't lost the last of his thinning hair and the hard eyes behind the small lenses of his glasses were just as fierce as ever. 'It's no one I've seen before,' I said, handing the picture back to Bower. 'Is he someone we've had contact with?'

'Not as far as I know,' said Bower. To Valeri he said, 'Describe the deliveries again.'

'The second Thursday of every month… The KGB courier.'

'And you saw him open it?' persisted Bower.

'Only the once but everyone knows…'

'Everyone?'

'In his office. In fact, it's the talk of Karlshorst.'

Bower gave a sardonic smile. 'That the KGB liaison is sniffing his way to dreamland on the second Thursday of every month? And Moscow does nothing?'

'Things are different now,' said Valeri adamantly, his face unchanging.

'Sounds like it,' said Bower, not concealing his disbelief.

'Take it or leave it,' said Valeri. 'But I saw him shake the white powder into his hand.'

'And sniff it?'

'I was going out of the room. I told you. I shut the door quickly, I wasn't looking for trouble.'

'And yet you could see it was white powder?'

'I wish I'd never mentioned the damned stuff.' I had him sized up now. He was a typical old-time Communist, one of the exiles who'd spent the war years in Moscow. Many such men had been trained for high posts in the Germany that Stalin conquered. What was the story behind this one? Why had he come to work for us? Blackmail? Had he committed some crime – political or secular – or was he not of the hard stuff of which leaders are made? Or was he simply one of those awkward individuals who thought for themselves?

'No comment,' said Bower in a tired voice and looked at his watch.

Valeri said, 'Next week I'll watch more carefully.'

I noticed Bower stiffen. It was a damned careless remark for an active agent to make. I was not supposed to discover that this Valeri was a double; going in and out regularly. It was the sort of slip of the tongue that kills men. Valeri was tired. I pretended not to have noticed the lapse.

Bower did the same. He should have noted it and cautioned the man but he gave an almost indiscernible shake of the head to the shorthand clerk before turning his eyes to me. Levelly he asked, 'Is that any use?' It was my signal to depart.

'Not as far as I can see.'

'Frank wanted you to know,' he added just in case I missed the message to get out of there and let him continue his difficult job.

'Where is he?'

'He had to leave.' Bower picked up the phone and said they'd break for lunch in thirty minutes. I wondered if it was a ploy. Interrogators did such things sometimes, letting the time stretch on and on to increase the tension.

I got to my feet. Tell him thanks,' I said. He nodded.

I went out to where Teacher was waiting in the ante-room. He didn't say 'All right?' or make any of the usual polite inquiries. Interrogations are like sacramental confessions: they take place and are seen to take place but no reference to them is ever made. 'Are you returning me to Kreuzberg?' I asked him.

'If that's where you want to be,' said Teacher.

We said our goodbyes to the Duchess and went downstairs to be let out of the double-locked front door by the guardian.

The streets were empty. There is something soul-destroying about the German Ladenschlussgesetz – a trade-union-inspired law that closes all the shops most of the time – and right across the land, weekends in Germany are a mind-numbing experience. Tourists roam aimlessly. Residents desperate for food and drink scour the streets hoping to find a Tante Emma Laden where a shopkeeper willing to break the law will sell a loaf, a chocolate bar or a litre of milk from the back door.

As we drove through the desolate streets, I said to Teacher, 'Are you my keeper?'