'I don't like the sound of it,' said Tessa. 'I thought there was something odd going on when Fiona asked me to look after her fur coat.'

'Is there anything for lunch?' asked George.

'There's a home-made chicken stew in the freezer.'

'Is that still all right? It's dated 1981.'

'I spent hours on that stew,' said Tessa, aggrieved that such rare forays into domesticity were not appreciated.

By the time that Tessa arrived at the Samson house, two heavily built men who answered to Bret were rolling up the overalls they had worn to probe between the floorboards and investigate every inch of the dusty attic. Bret Rensselaer was standing before the fireplace wearing a black trenchcoat. He finished the coffee he was drinking.

He'd recently seen Tessa at Whit elands, and without preliminaries said, 'Mrs Samson has taken a trip to the East.' He put his cup on the mantelshelf. 'For the time being the children need someone to reassure them… The nanny seems to be taking it very calmly but your presence could make all the difference.' Bret had insisted that Fiona engage a reliable girl who could survive a proper security vetting. The present nanny was the daughter of a police inspector. Now and again Fiona had complained that she was not a very good nanny but now Bret's caution was paying off.

'Of course,' said Tessa. 'I'll do anything I can.'

'We're very much in the dark at present,' Bret told her, 'but whatever the truth of it there will be no official comment. If you get any calls from the Press, or any other kind of oddball, say you are the housekeeper, take their number and call my office.' He didn't tell Tessa that every call to this phone was being monitored and two armed men were watching the house to make sure that Moscow didn't try to kidnap the children.

One of the children – Billy – came from the kitchen where Nanny was frying eggs and sausages for lunch. 'Hello, Auntie Tess. Mummy is on holiday.'

'Yes, isn't that fun?' said Tessa, leaning down to kiss him. 'We are going to have a wonderful time too.'

Billy stood there looking at Bret for a moment and then summoned up the courage to say, 'Can I look at your gun?'

'What's that?' said Bret, uncharacteristically flustered.

'Nanny says you have a gun in your pocket. She says that's why you won't take your raincoat off.'

Bret wet his lips nervously, but long before he could think of any reply, seven-year-old Sally appeared and grabbed Billy by the arm. 'Nanny says you are to come to the kitchen and have your lunch.'

'Come along children,' said Tessa. 'We'll all have lunch together. Then I'll take you somewhere lovely for tea.' She smiled at Bret and Bret nodded his approval and appreciation.

'I'll slip away soon,' said Bret. He'd heard somewhere that Tessa Kosinski had been using hard drugs, but she seemed very normal today, thank heavens.

In the dining room, Nanny was dishing up the food. She had set the big polished table for four, as if guessing that Tessa would eat with them.

After the two technicians had packed away their detection apparatus and left, Bret took a quick look round on his own account. Upstairs on Fiona's side of the double bed a nightdress was folded neatly and placed on the pillow ready for her. On the bedside table he saw a book from the Department's library. He picked it up and looked at it: a coloured postcard – advertising a 'hair and beauty salon' off Sloane Street – was being used as a bookmark. He stood there for a moment relishing the intimacy of being in her bedroom. From a security point of view there was nothing to worry him anywhere. The Samsons had worked for the Department a long time: they were careful people.

As he let himself out of the front door, Bret heard Billy insisting, 'Well, I'll bet he's shot lots of people.'

Bernard Samson had been arrested in a Biergarten near Müggelheimer Damm. It was a forest that stretched down to the water of the Müggelsee. A thousand or so inebriated men celebrating Himmelfahrt – Ascension Day – had provided the congestion and confusion in which Bernard, and his closest friend Werner Wolkmann, had helped two elderly refugees to escape westwards. It was not a simple act of philanthropy: one of the escapees was an agent of the Department.

Werner and the others had got away when Bernard created a diversion. It was a brave thing to do but Bernard had had ample time to regret his rash gesture. They had locked him in an office room on the top floor of the State Security Ministry's huge office block on Frankfurter Alice.

This office was not like the cells in the basement – from which some prisoners never emerged – but its heavy door and barred window, plus the difficulty of moving from floor to floor in a building where every corridor was surveyed by both cameras and armed guards, was enough to hold anyone but a maniac.

Bernard had been interrogated by an amiable KGB officer named Erich Stinnes. He spoke the same sort of Berlin German that Bernard had grown up with, and in many things the two men saw eye to eye. 'Who gets the promotions and the big wages – desk-bound Party bastards,' said Stinnes bitterly. 'How lucky you are not having the Party system working against you all the time.'

'We have got it,' said Bernard. 'It's called Eton and Oxbridge.'

'What kind of workers' state is that?' said Stinnes.

'Are you recording this conversation?' asked Bernard.

'So they can put me in prison with you? Do you think I'm crazy?'

It was the sort of soft treatment that was usually followed up by browbeating from a ferocious tough guy partner, but Stinnes was waiting for a 'KGB Colonel from Moscow', who turned out to be Fiona Samson from London.

By that time Bernard Samson had begun to suspect what was about to happen. Some of the clues that Bret Rensselaer had so artfully supplied to the other side had become evident to the ever more worried Bernard.

The desperate realization that his wife was a KGB Colonel was a betrayal of such magnitude that Bernard felt physically ill. But the effect upon him – and the agony of it – was not greater than many men have suffered when discovering that their wife has been unfaithful to them with another man. For each individual there is a threshold beyond which pain does not increase.

For Fiona the pain was made worse by the guilt of inflicting it upon a man who loved her. She was very tired – and the journey had left her with a splitting headache – that morning when they brought Bernard in to face her. It was a test – perhaps the toughest one she would face – of her ability, her conviction and her resolution to pursue her role even in the face of Bernard's contempt and hatred.

Brought in by a guard he was dirty and unshaven. His eyes stared at her in a way she had never seen before. It was a horrible hateful exchange but she played her part determined that Bernard would see no glimmer of hope. Only his despair would protect her.

There was a tray with coffee pot and cups on the desk but Bernard didn't want any. 'Is there anything to drink in this office?' he demanded.

She found a bottle of vodka and gave it to him. He poured it into a cup and drank a large measure in one gulp. Poor Bernard: she suddenly became afraid that this would be the beginning of a long drunken bout. 'You should cut down on the drinking,' she said.

'You don't make it easy to do,' he said. He smiled grimly and poured more for himself.

'The D-G will send for you, of course,' she said more calmly than she felt. 'You can tell him that the official policy at this end will be one of no publicity about my defection. I imagine that will suit him all right, after all the scandals the service has suffered in the past year.'

'I'll tell him.'

She watched him: he'd gone green. 'You never could handle spirits on an empty stomach,' said Fiona. 'Are you all right? Do you need a doctor?'