I chew my tongue, thinking I’ve nearly flipped the tumbler, but no dice. On to the next. ‘It depends. On the type of lock.’

‘Well, like, at the police station, how long did it take?’

‘Maybe five minutes? But that was an easier sort of lock. This one is proving a little trickier.’ On to the third.

‘What if you can’t open it?’

‘I don’t know, Ellis. We’ll figure it out.’

‘Well, should we m-m-maybe have thought about that before w-we –’

‘Please!’ I whip my head round to look at him and he deflates. ‘Ellis. I’m sorry for snapping, OK? But you’re making me nervous. Please trust me. What happened to the version of you who followed me into the police station? Totally unprompted, before any of this?’

‘That was different. There wasn’t a m-murderer on the loose then.’

‘If anything, the fact there’s a murderer on the loose should dial you in.’

Except I try the fourth and fifth and by then it’s been ten minutes and I’m out of picks.

I curse. ‘I don’t know what’s wrong.’ We can’t just leave. If everything we’ve learned is a funnel that’s coiled around us, this place is our only exit. The only way out is through.

‘You could try the front door,’ Ellis says.

But I’m looking somewhere else: the window. It leads straight into the office.

Ellis knows what I’m thinking.

‘M-Mona, no. No. Have you lost it?’

I don’t know whether or not I have, but I know my blood is boiling and it’s erupting and it needs out. I know whatever I have is less of a haunting and more of a possession – it’s not something outside me that wants to come in, but the other way round. I created this feeling and it needs somewhere to go. I didn’t always use to be the kind of girl who could do this but I know more than anything, more than a bullet to the brain, that I am now.

I palm a heavy rock from the building’s perimeter and experimentally toss it between my hands.

Then I chuck it at the window with everything I have.

Ellis yelps, but the window doesn’t even shatter – it bounces straight off and nearly hits me in the leg.

‘M-M-M-Mona,’ Ellis says. ‘I can’t do this. P-Please. I really, really can’t do this.’

I pick up the rock again, ready for a second go. My shoulder’s throbbing in the socket. ‘Then go,’ I say. ‘Go home. You don’t need to be here.’

He looks at me, hurt, and my chest crumples. ‘M-Mona …’

‘Go, Ellis. Go. Or shut up and let me do this.’

I regret the words as soon as I’ve said them but they’ll mean nothing if I don’t stand by them.