‘How did-’

‘Please.’ Straub silenced him with an emphatic, chopping gesture. ‘We do not have much time. I will tell you everything you need to know. Marty is in Hamburg. He is locked inside my mother’s apartment. He is tied to a chair with his mouth taped. He has been there’ – Straub consulted his watch – ‘for nearly twenty-four hours. My mother is away on holiday, you see. She will not be back until the middle of next week. So, no one will find Marty in time to save him from death by dehydration.’

‘You… must be joking.’

‘I am not. You can save him, Richard. In fact, only you can save him. I have a key in my pocket that will open a left-luggage locker at Hamburg central station. Inside the locker is a set of keys to the apartment, with a tag tied to them. The address is written on the tag. There is a train to Hamburg at twenty-one ten. It will arrive at one fifteen tomorrow morning. You should be on it. If your friendship with Marty means anything to you, you will be. Naturally, I require something in return for the key. I require the attaché case.’ Straub sat back and raised his glass. ‘Prost.’ He took a sip.

Eusden stared at him, unable for the moment to formulate a response. The man surely had to be mad to go to such lengths. Though perhaps he had not gone to such lengths. There was always a chance that this was merely a ruse to trick Eusden into surrendering the case. But why? What could the case possibly contain that made sense of all this?

‘Perhaps you do not believe me,’ said Straub, reading his mind with discomfiting accuracy. ‘If so, you will find this interesting.’ He took his phone out, pressed some buttons and held it out so that Eusden could see the screen. ‘A captured image that will dispel any doubts.’

Eusden squinted at the screen. And there was Marty, older and gaunter than he remembered, but still instantly recognizable by his mop of curly hair and the jut of his brow. He was dressed in jeans, sweatshirt and trainers and was sitting in an upright wooden chair, his ankles roped to the legs, his shoulders pulled back, his wrists bound out of sight behind him. There was a shiny smear where his mouth was covered by a strip of tape and a length of rope stretched taut from the back of the chair to an anchoring point out of shot. The setting appeared to be some kind of domestic interior. And the picture came complete with a timer display that proved it had been taken the previous night: 22:32, 04.02.07.

Straub withdrew the phone and slipped it back into his jacket. ‘The case, Richard. I must have it.’

‘You won’t get away with this.’

‘I think I will. Marty will not want you to go to the police. Take my word for that. Better still, ask Marty when you see him.’

‘He’s not a well man. You know he isn’t. How’s he going to stand up to the ordeal you’re putting him through?’

‘Go and find out.’

‘You’re a cold-hearted bastard, aren’t you?’

Straub looked as if he found the accusation faintly flattering. ‘The case, Richard. I will have it now, please.’

Eusden hesitated for a moment. But the simple if unpalatable truth was that he had no choice. He picked up the attaché case and handed it over.

‘Thank you.’ Straub laid it on the table in front of him. From his pocket he took a small key and unlocked the catches. Eusden could see nothing of the contents when he raised the lid. There was a sound of papers being riffled through, then Straub gave a frowning nod. ‘Good,’ he said, closing the case and relocking it. He smiled. ‘Excellent, in fact.’

‘You have your… collectable?’

‘I do.’

‘I hope you think it’s worth what you’ve done.’

‘There is no doubt that it is.’ Straub delved in another pocket. When he reached across the table and opened his hand, Eusden saw a different key resting in his palm, larger and chunkier than the one that had opened the case. ‘Locker number forty-three, Richard. Time you were going, I think. You need to catch that train. They empty the lockers after twenty-four hours. I engaged it this morning at eight o’clock. So, you need to be there before eight o’clock tomorrow morning. The twenty-one ten is the last train tonight. You cannot afford to miss it. I suggest you start for the station. Now.’

Eusden was unsure in retrospect how he endured the four-hour journey to Hamburg. The train was old and slow and grubby, the route a grim haul through industrial towns and stretches of countryside veiled in darkness. Most of the passengers looked about as happy to be aboard as he was. They were travelling, like him, because they had to.

Eusden had been sorely tempted to call Gemma and offload some of the concern he felt for Marty and the anger that filled him at being put in such a position. But there was nothing Gemma could do except worry. And it was not her fault that Straub had laid a trap for one of them to walk into. Eusden suspected it might at least partly be Marty’s fault, however, a suspicion he intended to voice once he was sure his friend had come to no harm. All he could do meanwhile was stare out at the night-blanked North German Plain and stifle his frustration.

Hamburg central station was thinly populated at 1.15 in the morning, a deadening chill invading its cavernous, empty spaces. Eusden, drained by sleeplessness and anxiety, tracked down the left-luggage lockers as swiftly as he could and opened number 43.

The keys were there, as Straub had promised. And so was the tag. The address written on it in bold block capitals meant nothing to him. He could only hope a taxi driver would be able to find it.

The night he walked out into was still and numbingly cold. He clambered into the lead cab in a short queue of more or less identical cream Mercedes and proffered the tag to the driver. A glance and a nod was all he received in return. Then they were on the move.

A ten-minute surge through a deserted city centre and they were there: Brunnengasse, a pedestrianized link-route between a main road and a residential side street. Modest but reputable apartment blocks lined the route, prettied up with window-boxes and Juliet balconies. The address on the tag was number six: a single door serving twelve flats, each equipped with electronic bell-pushes, speakerphones, and mailboxes next to the entrance. There was, however, no way of telling which flat belonged to Straub’s mother.

Eusden let himself in with one of the Yale keys and started checking the names displayed alongside the front doors of the flats. He had reached the third floor before he found what he was looking for: FRAU B. STRAUB. He rang the bell. There was no response. He tried again, pressing his ear to the door. Was that a muffled groan he heard? Maybe. Maybe not. He unlocked the door and pushed it open.

The picture on Straub’s phone had prepared him for what he would see. But what he could actually see was very little. The flat was in darkness, a wedge of amber lamplight from the street illuminating only a portion of carpet in a room ahead of him. There was that groan again, apparently emanating from the same room. Eusden groped for the light switch and pushed it down. Nothing happened.

Another groan, louder this time. He headed for the wedge of amber and entered what various hummocked shadows suggested was a lounge, with a pair of windows affording a view of the flats opposite. As his eyes adjusted to the gloom, one shadow resolved itself into a figure lying on the floor. It was Marty. He had toppled the chair over at some point, but was still tied to it, slumped on his left side.

‘It’s me, Marty,’ said Eusden, stooping over him. He caught a pungent whiff of stale sweat and urine. Marty turned his head and rolled his eyes. ‘Hold on.’ Eusden located one end of the strip of tape and pulled it free as gently as he could.