On screen, Ben shook his head. ‘Margery Allen saw me. I had to tie up loose ends, removing the remotest possibility anyone ever gets near the truth. David’s sudden disappearance would have raised questions, so I gave him closure. Probably the most difficult thing I’ve ever done. It doesn’t matter how many times I’ve used these transits; it takes a lot to convince yourself one will save you from a fatal fall. But it was necessary. The resemblance was good enough for Margery Allen to believe she’d seen David jump, so when the police showed her his photo, that’s who she thought she saw. Your investigation tied up all the loose ends.’

‘And if I don’t do this, I’ll create a paradox,’ Elliot remarked.

Harri sensed resignation in his voice.

‘It’s never not been done,’ Ben said on screen. ‘But if it were possible to break the chain of events, I imagine the consequences would be profound.’

Elliot hesitated. ‘How? How do I do this?’

‘You leave now,’ Ben replied.

A light came on further into the cave, illuminating an old backpack. Harri recognized it as the one Ben had been wearing the night Sabih had died. Next to the backpack was a sphere, one of the transits.

‘There’s a passport, identity papers, academic credentials, and bank cards in the name of Ben Elmys. The real Ben Elmys died in the car accident with his friends, but I altered the official record to show he was the sole survivor. This will be your name from now on.’

Elliot walked over to the backpack. He picked it up and pulled out something about the size of a shoebox. It was made of metal and covered in digital input panels, some of which were cracked.

‘This is how you make them, isn’t it?’ Elliot asked.

Ben nodded. ‘Yes. It’s called a forge. It creates the spheres. I call them transits. It was broken during my fight with Sabih. You can reverse engineer the technology to help you build a new one.’

‘The missing cobalt?’ Harri asked, gesturing at the device.

‘Yes,’ Ben replied on screen. ‘I took it from the university and used it to power the forge.’

Elliot picked up the transit that was next to the backpack. ‘And this?’

‘One of only three transits that now exist. The last three I created before the machine was broken. It will take you to Sydney, thirty-one years ago. Inside the bag you’ll also find all the letters I wrote to you from prison. The ones you destroyed. I wrote two copies of all of them, because I knew you would never read the ones I sent you. They will tell you everything you need to know about your path.’

‘What about me?’ Harri asked.

‘You’ll be given a choice, but not until Elliot leaves. He can’t know what you’re going to do. The temptation to look for a shortcut or an easier path would be too great. Moving in four dimensions requires paranoia about security and timeline constancy.’