That would seriously limit their movements.'

'I'll keep it to myself, of course. And so will my son.'

'I wish you would, Jan,' I said. 'It's very delicate, very delicate.'

'I hate Russians,' said old Jan.

Giles Trent's house was one of a terrace of narrow-fronted Georgian-style dwellings erected by speculative builders when the Great Exhibition of 1851 made Chelsea a respectable address for senior clerks and shopkeepers. Near the front door – panelled and black, with a brass lion's-head knocker – stood Julian MacKenzie, a flippant youth who'd been with the Department no more than six months. I'd chosen him to keep an eye on Trent because I knew he wouldn't dare ask me too many questions about it or expect any paperwork.

'He arrived home in a cab about half an hour ago,' MacKenzie told me. 'There's no one inside with him.'

'Lights?'

'Just on the ground floor – and I think I saw some lights come on at the back. He probably went into the kitchen to make himself a cup of cocoa.'

'You can go off duty now,' I told MacKenzie.

'You wouldn't like me to come in with you?'

'Who said I'm going in?'

MacKenzie grinned. 'Well, good luck, Bernie,' he said cheerfully, and gave a mock salute.

'When you've been with the Department for nearly twenty years and the probationers are calling you Bernie,' I said, 'you start thinking that maybe you're not going to end up as Director-General.'

'Sorry, sir,' said MacKenzie. 'No offence intended.'

'Buzz off,' I said.

I had to knock and ring three times before I could get Giles Trent to open the door to me. 'What the devil is it?' he said before the door was even half open.

'Mr Trent?' I said deferentially.

'What is it?' He looked at me as if I was a complete stranger to him.

'It would be better if I came inside,' I said. 'It's not something we can talk about on the doorstep.'

'No, no, no. It's midnight,' he protested.

'It's Bernard Samson, from Operations,' I said. Why the hell had I been worrying about Giles Trent recognizing me in the club? Here I was on his doorstep and he was treating me like a vacuum-cleaner salesman. 'I work on the German desk with Dicky Cruyer.'

I'd hoped that this revelation would bring about a drastic change of mood, but he just grunted and stood back, muttering something about being sure it could wait until morning.

The narrow hall, with Regency striped wallpaper and framed engravings by Dutch artists I'd never heard of, gave onto a narrow staircase, and through an open door I could see a well-equipped kitchen. The house was in a state of perfect order: no nicks in the paintwork, no scuffs on the wallpaper, no marks on the carpet. Everything was in that condition that is the mark of those who are rich, fastidious and childless.

The hall opened onto the 'divine' living room that Tessa had promised. There was white carpet and white walls and gleaming white leather armchairs with brass buttons. There was even an almost colourless abstract painting over the white baby grand piano. I could not believe it was an example of Giles Trent's taste; it was the sort of interior that is designed at great expense by energetic divorcées who don't take cheques.

'It had better be important,' said Trent. He was staring at me. He didn't offer me a drink. He didn't even invite me to sit down. Perhaps my sort of trench coat didn't look good on white.

'It is important,' I said. Trent had taken off the tie he'd been wearing at Kar's Club, and now wore a silk scarf inside his open shin. He'd replaced his jacket with a cashmere cardigan and his shoes with a pair of grey velvet slippers. I wondered if he always dressed with such trouble between coming home and going to bed, or whether his informal attire accounted for the delay before he opened the door to me. Or was he expecting a visit from Tessa?

'I remember you now,' he said suddenly. 'You're the one who married Fiona Kimber-Hutchinson.'

'Were you at Kar's Club tonight?' I said.

'Yes.'

'Talking to a member of the Russian Embassy staff?'

'It's a chess club,' said Trent. He went across to the chair where he'd been sitting, placed a marker in a paperback of Zola's Germinal, and put it on the shelf along with hardback copies of Agatha Christie and other detective stories. 'I speak to many people there. I play chess with anyone available. I don't know what they do for a living.'

'The man you were with is described in the Diplomatic List as a first secretary but I think he's a KGB man, don't you?'

'I didn't think about it, one way or the other.'

'Didn't you? You didn't think about it? Okay if I quote you on that one?'

'Don't threaten me,' said Trent. He opened a silver box on the table where the book had been and took a cigarette and lit it, blowing smoke in a gesture that might have been repressed anger. 'I'm senior in rank and service to you, Mr Samson. Don't come into my home trying the bullyboy tactics that work so well with other people of your own sort.'

'You can't believe that being senior in service and rank gives you the unquestioned right to have regular meetings with KGB agents and discuss the merits of various photocopying services.'

Trent went red in the face. He turned away from me, but that of course only drew attention to his discomfiture. 'Photocopying? What the hell are you talking about?'

'I hope you're not going to say that you were only going to photocopy chess problems. Or that you were meeting that KGB man on the orders of the D-G. Or that you were engaged on a secret assignment for a person who's name you are not permitted to tell me.'

Trent turned and came towards me. 'All I'm going to tell you,' he said, tapping my chest with his finger, 'is to leave my house right away. Any further conversation will be done through my lawyer.'

'I wouldn't advise you to consult a lawyer,' I said in the friendliest tone I could manage.

'Get out,' he said.

'Aren't you going to tell me that you'll make sure I'm fired from the Department?' I said.

'Get out,' he said again. 'And you tell whoever sent you that I intend to take legal action to safeguard my rights.'

'You've got no rights,' I said. 'You sign the Act regularly. Have you ever bothered to read what it says on that piece of paper?'

'It certainly doesn't say I'm not entitled to consult a lawyer when I have some little upstart force his way into my house and accuse me of treason, or whatever it is you're accusing me of.'

'I'm not accusing you of anything, Trent. I'm just asking you some simple questions to which you are supplying very complicated answers. If you start dragging lawyers into this dialogue, our masters are going to regard it as a very unfriendly reaction. They are going to see it as a confrontation, Trent. And it's the sort of confrontation you can't win.'

'I'll win.'

'Grow up, Trent. Even if you went to law and did the impossible and got a verdict against the Crown so that you were awarded damages and costs, do you think they'd give you your job back? And where would you go to find another job? No, Trent, you've got to put up with being quizzed by menials like me because it's all part of the job, your job. Your one and only job.'

'Wait a minute, wait a minute. There are a couple of things I want to get straight,' he said. 'Who says I've been in regular contact with this Russian diplomat?'

'We've got this funny system in interrogation – you wrote one of the training books, so you'll know about this – that it's the interrogator that asks the questions and the man being investigated that answers them.'

'Am I being investigated?'

'Yes, you are,' I said. 'And I think you are as guilty as hell. I think you're an agent working for the Russians.'

Trent touched the silk scarf at his neck, loosening it with his fingers, as if he was too hot. He was frightened now, frightened in the way that such a man could never be by physical violence. Trent enjoyed physical exertion, discomfort and even hardship. He'd learned to deal with such things at his public school. He was frightened of something quite different: he was terrified that damage was going to be done to the grand illusory image that he had of himself. It was part of my job to guess what frightened a man, and then not to dwell on it but rather let him pick at it himself while I talked of other, tedious things, giving him plenty of opportunity to peel back the scab of fear and expose the tender wound beneath.