‘Because she was being framed.’

‘That’s a theory,’ Hussein said. ‘But it’s not one we can usefully pursue until Dr Klein is in custody.’

‘Sandy was scared Frieda was in danger. Then Sandy was killed. Doesn’t that suggest you’re looking in the wrong place for the murderer?’

‘No. It suggests that we need to find Frieda Klein and question her.’

‘But –’

‘I appreciate your concern,’ Hussein said. ‘And I hope that you appreciate I’m not trying to stitch up your friend but to get to the truth. That’s my job. That’s what I intend to do. And that’s what is in everybody’s best interests, including Frieda’s.’

‘Of course,’ said Karlsson.

‘So are you going to help?’

‘What do you mean?’

‘Where is she? I assume that’s why Mrs Rasson came to you not me – because she thought you could let Frieda know she was in danger. I’m not entirely stupid.’

‘I never thought you were.’

‘So?’

‘I don’t know where she is.’

‘You had better be telling me the truth.’

‘I am. I don’t know.’

Karlsson didn’t know, but after he had spoken to Hussein he told Yvette he was going out for a while. Thirty-five minutes later, he was sitting in Reuben McGill’s office in the Warehouse. Reuben, his shirt-sleeves rolled up, sat on the sill of the open window and smoked.

‘Is this going to be awkward?’ he asked.

‘I’m concerned for Frieda’s safety. I need your help.’

Reuben threw his cigarette stub out of the window and turned towards Karlsson. ‘Is this a way of getting me to talk?’

‘Frieda’s in danger.’

‘Yeah, yeah.’

‘I’m here as Frieda’s friend. I’m not on the inquiry.’

Reuben looked at him through narrowed eyes. ‘What kind of danger?’

‘I don’t know. But Sandy was trying to warn her before he died.’

Reuben came away from the window and sat at his desk, his chin propped on his hands. ‘I don’t know what I can do,’ he said.

‘You don’t need to tell me where she is, but you need to tell her what I’ve told you.’

‘I don’t know where she is.’ He met Karlsson’s sceptical gaze. ‘It’s the truth. She’s disappeared.’

‘You have no way of getting in contact with her?’

‘No.’ He unfolded his hands so that they covered most of his face and closed his eyes. Karlsson waited. ‘You swear you’re not tricking me?’

‘I’m not tricking you.’

Reuben spoke slowly, reluctantly. ‘I don’t know why I’m saying this. But if anyone knows anything, Josef does. I may have done a terrible thing telling you that.’

‘I won’t get him into trouble.’

‘Frieda would never forgive you.’

‘Where is he now?’

‘He’s working at a house in Belsize Park. He’s stubborn. As you know.’

‘We’ll see.’

Reuben nodded and wrote down the address on a piece of paper that he tore from the pad and handed across the desk. ‘If this goes wrong,’ he said, ‘I’ll come for you with all the weapons in my psychotherapeutic arsenal.’

‘I’ll remember that,’ said Karlsson. He took the paper and left.

He found Josef in the back garden of the house. He was with a group of men, drinking tea, smoking. Josef saw him and rose to his feet, looking wary. ‘Nothing to say.’

Karlsson took him by the arm and led him away from the group of men, who were watching them curiously. ‘There’s something you should know.’

‘You think you scare me?’

‘I’m not going to threaten you.’ He held up a hand to stop Josef interrupting. ‘I’m not going to ask if you know where she is. I’m just giving you this.’ He thrust his hand into the inside pocket of his jacket and pulled out the letter he had written in the café down the road.

Josef stepped away from it as though it were a bomb that might explode in his face. ‘This is trick.’

‘What trick could it be? I am giving you a letter. It would be good for Frieda if she read it, but that’s up to you.’

‘I know nothing.’

‘Then I’m wasting my time.’ He waited a moment. ‘I’m Frieda’s friend and I have reason to believe that she’s in danger.’

‘You are police.’

‘That too. But you can still trust me.’

Josef wrinkled his face, which was grimy. There was dust in his hair and Karlsson saw that his hands were blistered.

‘You say danger,’ he said.

‘Yes.’

Josef glowered at him. ‘If I take it, then it means nothing.’

‘OK.’

He held out the letter once more and this time Josef took it. As soon as Karlsson had gone, Josef pulled out his phone. Frieda had given him her new number. He dialled it. No answer.

Frieda felt as if she was on the verge of saying goodbye to Ethan, for the moment anyway. This couldn’t go on. They got onto a bus and went upstairs to the front. Ethan stood up on the seat and stared out of the window and gave a running commentary on what he could see: people and pets and cars and bikes and houses and shops. The bus went through Elephant and Castle and down the Old Kent Road. They got out and Ethan said he was tired and that he was hungry.

‘Wait,’ said Frieda.

She took him by the hand and led him off the main road and to the right and there, improbably, as if by magic, was something Ethan had never seen before. She led him through the gate, across the cobblestones, into the stables. Two horses peered out of their stalls, looking curiously at them. Frieda lifted Ethan up.

‘You can touch,’ she said. She put out her free hand and stroked the soft, salmon-pink skin between one of the horses’ nostrils. Ethan shook his head and leaned away. He didn’t dare touch the horses but he didn’t want to leave. Even when Frieda led him back out onto the pavement, he stared back behind him, as if he thought the stables might vanish when he stopped looking at them. Then they walked past the forge. Frieda tried to explain what a horseshoe was. Ethan just frowned. Frieda couldn’t tell whether he didn’t understand what she was saying or whether he did understand but didn’t believe it.

They continued following the tell-tale slopes and banks. Frieda noticed a broad pipe crossing the railway line. A few minutes later, she led him off into a little side-street. On the ground there were two manhole covers.

‘Do this,’ she said, and knelt on the ground and put her ear to one of them. He copied her. ‘Can you hear it?’ she said.

He sat up and nodded.

‘Do you know what it is?’ she said.

He shook his head.

‘Long, long ago there was a river,’ she said, ‘a little river. It ran through the streets and there were boats in it. And the horses, like the horses we saw, the horses drank from it. But then they hid the river. They covered it and built houses and roads on top of it. And people forgot about it. But the river is still there.’ She rapped on the metal cover. ‘That’s it, down there. It’s called the Earl’s Sluice.’

‘Sluice,’ he said solemnly.

‘That’s it. Only you and me know that it’s there and we won’t forget it, will we?’

‘No,’ he said obediently.

She stood up and held out her hand.

When they reached the Thames, Ethan put his head against the railings, as if he were trying to get at it. He seemed hypnotized.

‘This way,’ said Frieda, leading him westward along the riverside path. After a few hundred yards, when Ethan was starting to weigh on her, dragging on her arm to signal his tiredness, she bent down and spoke to him in a whisper. ‘I’ve got a surprise for you,’ she said.

‘What?’

She led him through the little gate into the city farm. When Ethan saw the goats and the cockerel and the rabbits, he looked as if cognitive overload were going to cause him to spontaneously combust. At first he simply stood still, with his mouth open. Then he started to run around, pointing at this animal, then that one and then another. After a while, Frieda took him to the café and bought him an ice cream, but he was restless, then started to cry and say he wanted to go back to the animals. So Frieda took her coffee, walked out and watched him as he went back into the enclosure.