‘I was scared for you,’ Kat explains. ‘Once you’d put that post up on MyNeighbour, risked exposing Beth’s secrets, I thought you could be in danger. I tried to warn you off, told you Beth didn’t like you, so that you’d distance yourself from her. But you didn’t listen. Like Mark didn’t take any notice when I told him to back off either.’

‘This is all fantasy, darling,’ Beth pleads. ‘Don’t listen to her.’

Saskia’s face curls into a grimace, but with pain not confusion this time, the aftereffects of her anaphylaxis. She closes her eyes, takes a few breaths, then steadies herself. ‘If you were watching Beth at Mark’s squat, like you say, why didn’t you stop her attacking him? Even if you’re telling the truth, that’s cruel in itself.’

Kat’s face crumples, an involuntary reaction to Saskia’s stinging words, because of course she’s right. Yes, Kat phoned for the ambulance, but she should have done more than that; she should have stopped Beth before one was needed. But instead, she froze. Petrified. Still cowed by Ellie’s threat after all this time. ‘Eighteen years ago, Beth made me believe I had no voice, no power,’ Kat explains, her voice pleading. ‘She told me that everything I do will always fail. That’s why I took the photos, made the voice recordings; I needed real evidence to back up what I knew before I could contact the police. But she was right; I was failing. When I was too scared to stop her attacking Mark, I realised it had to stop; I needed to be braver. That’s why I came here tonight.’

‘But I still don’t understand,’ Saskia exhales. The colour has drained from her face again, and there’s a film of sweat across her cheeks. ‘What have you known all this time?’

Kat turns towards Beth and sees a growing realisation cross her beautiful face, her fiery eyes turning to ash. Finally she’s worked out who Kat is. ‘I met Beth in a mental health clinic when we were teenagers,’ she says. ‘Although she was called Ellie then. She had anxiety disorder and mysophobia, a fear of germs.’

‘She’s lying, she’s crazy,’ Beth mumbles, scratching her palms.

‘I thought we were friends, until the night Beth admitted to murdering her brother.’

‘What?’ Saskia spits out.

‘He was a heroin addict. But he died of a stroke, an air bubble in the syringe. Everyone thought it was an accident, but when we were in the unit, Beth told me she did it.’

‘No, I would never,’ Beth whispers, too quietly to drown out the sound of her fingernails scratching. ‘It was an accident.’

‘You had a brother?’

‘And then you told me that her parents died in a car accident,’ Kat goes on, her voice starting to break. ‘And I’m scared she caused that too.’