Al didn’t know who she was and she was at a loss to explain herself to him. ‘I’ve been thinking about Sandy’s murder and some things have come up.’ She was conscious of Al’s nearly colourless eyes on her face as she spoke, and of the tameness of her words.

‘I’m confused,’ said Al, pleasantly. ‘You’re a nanny, right? Our nanny. At least, you were.’

‘Yes.’

‘And, for some reason, you want to ask me something about Sandy because you’ve been thinking about his death.’

‘I know about you and Veronica Ellison,’ said Frieda, suddenly. She’d had enough of this charade.

‘I beg your pardon?’

‘I said, I know about you and Veronica Ellison.’

He stared at her and she stared back.

‘I’m not even going to answer that,’ he said at last.

‘Sandy had some kind of an affair with Veronica, and then you did.’

‘Your point being?’ he asked. His voice was still perfectly polite.

‘I wondered if Sandy knew about it. Or Bridget.’

‘Did you now?’

‘I can’t ask Veronica. She’s on holiday and not answering her phone. I thought you could tell me.’

‘Are you quite mad?’ he asked. He didn’t say it rudely, more in a tone of amazement. ‘Why on earth should I tell you anything at all about my private life?’

‘Because it might help me understand why Sandy died.’

Al reached into the pocket of his running shorts and drew out a miniature iPod wrapped in its headphones. He started painstakingly untangling it.

‘Does Bridget know?’ repeated Frieda.

He looked up, resting his eyes on her with an expression of disdain. ‘No, she does not. And I hope she never will – unless, for some reason that I don’t pretend to understand, you think it fit to tell her.’ He gave her a curious little smile. ‘Of course, you will have to do what you think is right.’

Frieda thought of the passionate love letters from long ago that she had found in the locked tin in Bridget’s study. But it wasn’t the beautiful Bridget who had the secret to hide, it was her studious, gangly husband. She felt sick with herself but nevertheless asked the next question.

‘Did Sandy know?’

‘I’ve no idea. I assume not. Who would have told him? And what makes you think you have the right to ask me these questions? And now I’m done. And you, my friend, are likely to get into trouble if you go around asking questions like that. Everyone isn’t as understanding as me.’ He put the little buttons into his ears, shutting her off, gave her a nod, turned his back on her and broke into a slow trot.

That afternoon, Frieda took Ethan to the park. He was in high spirits: he hurled bread at the ducks, and at the playground tumbled from slide to seesaw to swing, where she pushed him high into the air and he screamed in joyous fear. As she lifted him out again and he collapsed into the buggy, she looked at the little boy’s face, in which she could see both Sasha and Frank. She would miss him, she realized. She had got used to the way he slid his hand into hers or fell asleep on her lap with a suddenness that always surprised her.

She gave him his beaker of juice and a biscuit and pushed the buggy out of the park onto the road that led towards Sasha’s house. It was a muggy, overcast day and she was thinking about Karlsson’s letter. She wondered about his children, Bella and Mikey, who had lived in Spain for a long time with their mother and stepfather. She remembered how painfully Karlsson had missed them. He had described it to her as a sharp pain, like something gnawing at him. As she was thinking this, a few drops of rain fell from the sky and there was a low rumble in the distance. She quickened her pace, hoping to get back to the house before the storm. And then she saw the group of young men, boys, really, a few yards ahead of her down the hill, shouting and jostling. It took her a few moments to realize that a figure was lying on the ground in their midst, a man with a thick beard, matted grey hair, grubby clothes. They were taunting him, laughing. One of them picked up an empty beer can and threw it at his head, and from where she stood Frieda heard him cry out in a high, wavering voice. She saw that other people were looking as well, furtively, not wanting to be involved. Rage, which felt pure and clean after the shameful encounter with Al, rose up in her. She bent down and fastened the safety straps around Ethan, who looked at her with his bright eyes.

‘Ethan, I’m going to run as fast as I can and you’re going to shout as loudly as you can. Your biggest scream. OK?’

‘Now?’

‘Now.’

He opened his mouth very wide and emitted a howl that hurt her ears. She took a deep breath and sprinted down the hill towards the group of youths, the buggy bumping wildly as she went. Ethan’s roar became a shriek. The buggy smashed into the first figure and Frieda caught a glimpse of a pimply, startled face. She veered into the next, lifting a fist and aiming it at him. She felt flesh against her knuckles, heard a grunt of pain. The figure on the ground was huddled into a foetus shape, all his pitiful things scattered around him. She swung round again and drove the buggy into a boy in a hoodie, who was staring at her with his mouth open slackly, in an expression of comic surprise.

The group was breaking up. People were arriving from across the street. The man stirred, lifted his head. She saw that he was crying.

‘Christ,’ said a voice, excitedly. ‘You were terrific. Just terrific. How did you do that?’

‘I’ve called the police,’ said another voice. A man came towards her, his mobile in his hand. ‘Someone will be here any minute. I got some of it on my phone.’

‘You can stop screaming,’ Frieda said to Ethan, although the sounds he was making were hoarse and intermittent now.

‘They just ran,’ the man said to Frieda. ‘I should have helped you. But it happened before I had time.’

‘Time to film it,’ a woman said.

‘It’s OK,’ said Frieda. ‘I’ll be on my way now.’

‘But the police will want to talk to you.’

‘You can tell them what happened. You saw it.’ She looked towards the man on the ground, homeless and now beaten up. ‘Make sure he’s OK. Buy him a drink, talk to him.’

‘But –’

Frieda left, pushing the buggy rapidly back up the hill. By the time she reached the top, Ethan had already fallen asleep.

‘I think my child-minding days are coming to an end,’ she said to Sasha, later that evening.

‘You’ve done too much already. I’m interviewing several nannies this week. I’m sure one of them will be fine. I’ve got several days’ leave I can take.’

‘I can do a few more days.’

‘You’ve done enough. I don’t know how I would have managed without you. Ethan will miss you. And so will I.’

‘Right,’ said Frieda. ‘Let’s talk about your story, if anyone asks questions about this.’

Walking away from Sasha’s house, Frieda saw Frank coming towards her. It was too late to cross the road or turn aside, so she just kept moving steadily forward, keeping her expression unconcerned. He seemed tired and sad, his dark brow furrowed. And he stared right through her without seeing her, as though she didn’t exist. Which was sometimes what she felt herself.

‘Look at this,’ said Yvette Long, flinging a newspaper onto Karlsson’s desk.

He picked it up. ‘All right,’ he said. ‘An active citizen. Good for her.’

‘You’re not looking closely enough.’

He glanced at the headline – ‘Have-a-Go Heroine’ – and then read the story about a woman with a buggy charging at a group of young men who were assaulting a homeless man. And then at the blurred photograph that showed a woman with very short dark hair, wearing bright clothes, running with a buggy.

‘Fuck,’ he said.

‘That’s what I thought,’ said Yvette. ‘And somebody filmed it with their phone. It’s on the website.’

‘Show me.’

Yvette went to her desk and tapped on a keyboard. ‘Here,’ she said.