The atmosphere is thick with despondency. I know that the men and women gathered in this incident room have worked day and night to try and find something to link the three victims, other than the manner in which the killer left their bodies. But it’s been weeks since Steph was discovered and nobody has been able to present a tangible solution.

The DI looks exasperated. ‘Come on, people! There must be one of you who can offer a potential insight. You’ve lived and breathed the victims’ lives for days. Why did he choose them? ANYBODY?’

My hand is tentatively rising before I realise it’s happening. Her eyes are wild as she looks over to me. ‘Amy?’

I feel the eyes of the room fall on me, and my cheeks burn with embarrassment. As I open my mouth to speak, it’s dry and the first syllables are unintelligible. ‘Online dating, ma’am,’ I eventually repeat.

Her eyes narrow, and I’m sensing that she is going to chastise me for making such a stupid suggestion. But when she speaks, there is a hint of excitement in her voice. ‘Go on.’

I reach for the pages behind me, eager to show that the idea isn’t totally insane. ‘We know that Steph Graham went on a date the night she died, and we’ve ruled him out. But what if she was followed home by someone else from the site? Maybe someone she dated and spurned, or maybe she wasn’t even aware of his interest.’ The pace of my voice is increasing as I speak. ‘Roxie O’Brien’s housemate said she’d been on a couple of blind dates in the weeks prior to her death. She couldn’t remember the name of the site Roxie used, but the lab boys have her computer and could probably confirm what’s in her internet history.’

The DI’s face begins to drop as she identifies a flaw in the theory. ‘Willow had only just broken up with her ex; she wasn’t known to be dating anyone else.’

I’ve already crossed this bridge in my mind, and I’m relieved that I prepared an answer. ‘Not so, ma’am. In his preliminary interview, the ex-boyfriend mentioned he was aware that Willow had been seeing several other guys, but he was convinced she’d come back to him… She had a dating app on her phone.’

The DI snaps her fingers and points at me as her eyes survey the faces of the rest of the team. ‘This is the kind of out-of-the-box thinking we should be doing. I want you all following up on this. Did they all use the same site? I want the names of every person who interacted with the victims on those sites. But don’t stop at names. Check for common dates of birth, addresses, bank accounts, profile images. Our killer may have multiple identities to protect himself. I want a list of potential suspects as soon as possible. I don’t care how large the list is initially. We will systematically find every person on that list and check their alibis and whereabouts on each of the nights. He will not kill again. We won’t let him.’

I look away as a couple of the old hands snarl in my direction, but the thumbs-up the DI flashes my way is gratefully received.

It’s the first time since I saw Steph Graham’s body in Finsbury Park that I don’t feel so scared. He is no longer the monster under my bed. Instead he is a man that I know will be found. The DI has said on countless occasions that she’ll never stop until she finds him, and there isn’t a single doubt in my mind that she’ll do it.

‘Amy,’ she calls me to the DCI’s office. ‘I want you with me when I visit Willow’s parents. The app on her phone was a good spot, but we need to know for certain whether she’d actually been on a date, or had been thinking about it.’

I close the door to the office to shut out the rest of the team. ‘Ma’am? I wasn’t going to say anything, but I overheard you and the DCI the other night. It was late and I was just logging off.’

‘And you want to know why I’m so convinced the same man is responsible for all three attacks?’

I shrug pathetically. ‘It’s not that I doubt the logic, it’s just—’

But she raises her hand to cut me off. ‘You want to know why I have no doubt we’re looking for one killer. Instinct. I feel it in here,’ she rubs her gut. ‘And I know it here,’ she taps a finger against her head, ‘and I’ve never been more certain of anything. If you’re to realise your potential and become a brilliant detective, you need to start listening to that voice in the back of your mind. We all have it. Some ignore the voice, but others embrace it.’

She must still see the doubt in my eyes.

‘You know, you remind me a lot of myself,’ she continues. ‘I once questioned every decision I made, wondering if I’d ever become the cop I aspired to be. It’s been a long road, and it hasn’t always been a smooth ride, but if you want to succeed in this place, you need to develop a fighter’s spirit. Your dad had that spirit. He put his life on the line for this job, and he’s a hero in my eyes because of that. Doing this job well isn’t about the ratio of cases you close or the number of arrests you make; it’s about the things you’ll do to keep the streets safe, no matter the cost.’

The DI pauses and grabs her coat. ‘Now, shall we go and take the first step towards proving your theory?’