He clicked the ‘play’ button. Things jerked and blurred and then came into focus. There was a youth with his mouth open wide throwing something and then a figure shot into the frame: a woman running and some unearthly noise coming from the buggy she was pushing in front of her, like a battering ram. For a moment she disappeared as another shape passed in front of her, the face out of focus, and then there she was again, her back to the camera. Then the film stopped. It had lasted about twenty seconds.

‘It could be her,’ he said.

‘It is her.’

He looked again. Yes. And he had a pretty good idea of who had been in that buggy. ‘Bloody Frieda,’ he said, but he felt oddly elated.

A few miles away, there was a call for Commissioner Crawford.

‘It’s from Professor Bradshaw,’ his assistant told him. ‘It’s something to do with Frieda Klein.’

When Sasha opened the door, she didn’t just look nervous, she looked distraught.

‘I’m Detective Chief Inspector Sarah Hussein. This is Detective Constable Glen Bryant. Can we come in?’

Sasha didn’t reply. She flicked her hair away from her face.

‘Are you all right?’ said Hussein.

‘Things are difficult,’ said Sasha. ‘I’ve got a little boy.’

‘We know.’

‘And I’ve just lost my childcare, which is irritating.’

Hussein and Bryant looked at each other.

‘Can we come in?’ said Hussein.

Ethan was sitting at a miniature red plastic table drawing with crayons in broad strokes, red and black and brown.

‘What is it?’ said Hussein, but Sasha picked him up before he could answer and sat on the sofa with him on her lap. He started to wriggle and to grab at her hair.

‘I need to put him in his room,’ said Sasha. ‘It’s time for his sleep.’

‘We can wait,’ said Hussein.

Bryant walked around the room, looking at the bookshelves as Ethan’s protesting cries receded upstairs. He ran his finger along the mantelpiece and inspected it. ‘The house needs a bit of a clean,’ he said.

Sasha came into the room and sat back down on the sofa. Faintly, from upstairs, there was the sound of wailing.

‘So he’s not quite asleep,’ said Hussein.

‘He doesn’t like sleeping,’ said Sasha. ‘Even when he’s tired out of his skull.’

‘What’s he like at night?’

‘The same. I haven’t had a proper night’s sleep for what seems like my whole life.’

‘I’ve been through that,’ said Hussein. ‘You need to leave him to cry and he’ll go to sleep.’

‘I’ve never been able to do that.’

Hussein nodded at Bryant. He took a photograph from the folder he was carrying and handed it to Sasha.

‘That was taken the day before yesterday near Clissold Park,’ he said. ‘A woman intervened in an assault.’

‘That sounds like a good thing to do,’ said Sasha.

‘She left the scene before the police arrived,’ said Bryant. ‘The media are calling her a have-a-go heroine. They’re looking for her. So are we.’

‘Why are you showing it to me?’

‘Look more closely.’

‘Why?’

‘Do you think she looks like Frieda Klein?’ said Hussein.

‘It’s a bit blurry.’

‘People who know her think she does.’

‘But why are you asking me?’

‘This mysterious heroine was pushing a buggy.’

‘Well, then,’ said Sasha.

‘What do you mean, “Well, then”?’

‘It can’t be Frieda.’

‘Unless she was looking after someone else’s child,’ said Hussein. ‘And, after all, it’s quite good cover, isn’t it? London’s full of people pushing buggies around. People don’t notice them.’

Sasha didn’t reply. She was scratching the back of her left hand as if she had an itch on it. This was the moment that Frieda had talked about. It seemed like years ago. They had rehearsed what she would say.

‘We’ve talked to people who know Frieda or work with her,’ said Hussein. ‘And you’re the only one with a young child. Why aren’t you at work?’

‘I told you. I’ve got a problem with my childcare.’

‘Who was looking after your child the day before yesterday?’

‘He’s called Ethan.’

‘Who was looking after Ethan?’

‘The nanny.’

‘Can we talk to her?’

‘She’s gone.’

‘Gone where?’

‘Back home. To Poland.’

‘To Poland. What’s her name?’

‘Maria.’

‘Maria what?’

‘I don’t know.’

‘You had a woman looking after your child and you don’t know her second name?’

‘I was in a crisis, my other nanny had suddenly left. I’d met her in the park. She said she’d stand in for a while. But now she’s gone as well.’

‘Maria from Poland. Was she connected with an agency? Do you have her bank details?’

‘I paid her in cash. I know you’re not supposed to, but everyone does it.’

‘Do you have a phone number for her?’

Sasha took a piece of paper from her trouser pocket and handed it over. Hussein looked at it. ‘She probably used a pay-as-you-go phone?’

‘Probably,’ said Sasha.

‘Would Ethan’s father confirm your childcare arrangements?’

‘We’re separated. He leaves it to me mainly. He doesn’t really know what’s going on day to day.’

‘He’s a barrister, is that right? Frank Manning.’

‘That’s right.’

‘Has he talked to you about your friend, Frieda? About the legal implications?’

‘No, he hasn’t.’

‘Many people don’t realize how serious it is to interfere with a police inquiry. A person who is caught and convicted will go to prison. Do you understand that?’

‘Yes.’

Hussein leaned in more closely and put her hand on Sasha’s elbow. ‘I know about you and Frieda. I know that she has helped you in the past and that you owe her a debt of gratitude.’

She saw that tears were running down Sasha’s cheeks. Sasha took a tissue from her pocket and blew her nose. Hussein felt so close. Just another push.

‘This insane behaviour cannot continue,’ she said. ‘The best thing you can do for your friend is to help us to find her.’

Sasha shook her head. ‘No,’ she said. Her voice was surprisingly firm. ‘I don’t know. I can’t help you.’

‘Do you understand what you’re risking?’ said Hussein. ‘You could go to prison. You’d lose everything. You’d be separated from your son.’

‘He’d probably be better off without me.’

‘Miss Wells. Do you expect us to believe this story? We can check it.’

Sasha wiped her face with her tissue. ‘I’ve told you everything I know. Check all you want.’

‘All right,’ said Hussein. ‘We’ll go through it one more time. And in more detail. And after that, we’ll go through it again. We have plenty of time.’

After Hussein and Bryant had gone, Sasha walked upstairs to Ethan’s room. He was asleep. She leaned down as she always did to check that he was still breathing. Sometimes she was so anxious that she woke him up to make absolutely sure, but this time he shifted slightly and gave a small whimper. Then she walked downstairs, picked up a phone and went out onto the little patio at the back of the house. She dialled a number and heard the click of it being answered.

‘Frieda?’

‘I’m here, Sasha.’

‘The police came to my house.’

‘I’m so sorry.’

‘It’s all right. I repeated what you told me to say.’

‘I don’t mean that. I put you at risk. I put Ethan at risk.’

‘You saved me and you saved him as well.’

‘This will be over soon,’ said Frieda. ‘For you as well as for me.’

‘That’s what I was ringing about. In a way. I need to tell you something.’

‘What?’

‘I can’t tell you over the phone. This needs to be face to face.’

‘That’s a bit awkward at the moment.’

‘I have to see you.’

Frieda paused for a moment. ‘All right. Where?’

‘There’s a place on Stoke Newington Church Street. It’s called Black Coffee. Can we meet there at half past ten tomorrow?’

‘Have you got anyone to look after Ethan?’

‘Frank’s coming round this afternoon. He might be able to take him. Or I’ll bring him along. He’ll be glad to see you.’