‘Hurt your hand?’ I asked, impatience gritting my teeth together.

‘Fractured a knucklebone punching that idiot.’ Demus winced. ‘Totally worth it.’ He took a bite of biscuit. ‘Anyway, as I was saying, there are two reasons why every past moment isn’t besieged by time travellers. The first reason is a practical one. It’s not the real reason but it’s interesting. The thing is that it takes a vast amount of energy to travel through time; the sort of thing that requires a nation to put some effort in. And the theory is really complicated. I mean really. It will take you two decades to figure your way through it, and that’s after I’ve sketched out the solution for you. But – and this is the important bit – it requires very little energy to create enough temporal micro-distortion to make that nation-sized effort fail. A small generator can make it impossible for thousands of square miles around it. And the theory behind the disruption is much easier. So, the cure is always discovered before the disease, and so if any government were worried that history might be rewritten by another government or that criminals might escape into the past . . . they build a few dozen disruptors as a precaution against new discoveries, and that’s an end to it.’

‘What the hell!’ I pointed to the two phantoms running straight at us across the mist sea: Mia and me, terror on our faces. And behind us, sprinting with deadly intent, someone with a long blade . . . a machete! ‘Is that . . . Rust? Ian Rust?’

The phantom Mia ran through us in a cold wash. Demus winced. ‘It’s an echo of a possible future.’

‘Well, fuck that!’ I raised my arms as the ghostly maniac ran at us and then broke into swirls.

‘Pay attention.’ Demus clicked his fingers. ‘The second reason. The real reason that we’re not deluged in time travellers is this: the arrival of the time traveller is an event like any other and branches a new timeline off from the reality he went back to. So, it’s never crowded. And because his arrival creates a new branch he can do all those paradoxical things you hear about. He can kill his father as a little boy to prevent his own birth. He can meet himself. It won’t matter, because he isn’t affecting the timeline that leads to him, he is changing events on a new timeline.’

I nodded, still watching the mist nervously, then thought of the message I had scratched onto Simon’s table. The one that had never reached me. ‘So it’s pretty useless to the traveller themselves, then. Because nothing they do has any bearing on the world they came from. You could go back and kill Hitler, but it wouldn’t save anyone you knew.’

‘Bingo.’ Demus put a finger to his nose and pointed at me with the other hand. ‘There are altruistic arguments to say you might want to kill the bastard anyway, and give another possible world a better deal . . . but we don’t care about that, do we?’

‘I guess not . . .’ In a universe where everything happened somewhere, every good thing, every bad thing, it seemed pointless to care about anything but the world you were given. ‘Is that—’ Out on the field phantom Mia was wrapped in an embrace with . . . I couldn’t say for sure. Now that I stared at them I wasn’t even sure it was Mia. The figures were breaking apart, becoming hard to see. ‘Was that—’

‘So.’ Demus waved the question away and leaned forward to stare at me. ‘You know who I am?’

I did, though I felt silly saying it out loud. ‘You’re me. A me who survived leukaemia, but somehow never grew his hair back.’ His face was proof enough. Now I saw it I couldn’t un-see it. Two or three decades stood in the way, but we were twins. More than that, now I looked closely, I saw that he even had the faint, white seam of scar on his forehead where I had head-butted the coffee table at age two. ‘You’re a me . . . And, bizarrely, I’m as pissed off about the hair as I am pleased about the living . . . though common sense does say that one should throw the other in the shade.’

‘I am.’ Demus nodded. ‘Technically it was our knucklebone I broke.’ He leaned back and surveyed the park. ‘It’s more remarkable than just me being any old time traveller, though. I’m a you who remembers all this. A you that really shouldn’t happen. I remember sitting where you are and having this conversation from your side. I remember me telling . . . me . . . about those dice rolls. You have to keep that piece of paper for the rest of your life, by the way. Or at least remember the numbers.’

I patted my pockets, suddenly terrified I’d lost it already. ‘I can’t—’

‘It’s under your pillow.’ Demus grinned. ‘So . . . I remember all this, and as long as I play my part, then what we do here really will impact my world. Our world.’

‘Play your part?’

‘I’m working to a script here, Nicky boy. I’m telling you what I remember me telling you. If I do something that I don’t remember doing – say I threw you on the ground right now, or shot you – or even if I don’t do something that I do remember doing – say I didn’t explain this bit . . . Well, that could happen, there’s nothing to stop it, but then we’ve branched away from my reality and I’ve lost my chance to make any difference.’

‘If that happens . . . If you killed me, for example, then how do you explain your memories?’

Demus shrugged. ‘Worst scenario would be that they’d be part of a genuine and dangerous paradox that could cause problems for the timeline as a whole. More hopefully they could be put down to madness, delusion, false memories created by a broken mind desperate for a solution to its woes. Even now, while we’re still on track, all of those things are way more likely than an actual closed loop where I can come back and change my future. That should be impossible. But it seems to have happened. Maybe the right number of stars went supernova all at once and tied an impossible knot in space-time. Who knows? It should be impossible, but here I am, making my own memories.’

I tried to focus, but whatever this resonance he talked about was it was making it difficult to just stay upright and not vomit.

‘OK, then why are you here? And can I have the cure for leukaemia that you must have brought back with you?’

‘Sorry, Nick.’ And to be fair, he did look sorry. Also pale and sweating, as if he was suffering, too. ‘It turns out that unravelling the fundamentals of the universe is easier than stopping the human body self-destructing. Einstein had nailed the theory of special relativity and given us E=MC2 on zero budget before people could listen to the radio, before the Wright brothers got a patent on their flying machine, and decades before antibiotics. By the time you’re my age you’ll have seen smart phones, the internet, and watched robots crawl over Mars, but we still won’t have cured the common cold. Or cancer.’ Demus finished his biscuit and wiped his mouth. ‘It may sound creepy because I’m forty and she’s fifteen . . . but I’m here for Mia, and I need you to get her to trust me.’

CHAPTER 8

Dizziness had swamped me and Demus had left before I collapsed. I’d started feeling better almost as soon as I lost sight of him.

It felt strange to know that the leukaemia wouldn’t kill me. I should have been elated, jumping for joy, albeit like an old man. But instead I didn’t feel anything really. Just burned out and empty. I still had to endure the treatment, the symptoms and the side effects. And as far as I understood things, if Demus’s game didn’t play out exactly the way he remembered it, then my survival would be up for debate again, too. The world would branch and I would no longer be the me who lived to become him.

The whole time travel claim seemed both ridiculous and, at the same time, the only possible explanation. And honestly, nothing had seemed quite real since Dr Parsons had sat me down to tell me that I had cancer two weeks earlier. A part of me had immediately started to expect a film crew to jump out from behind the curtains shouting, ‘You’re on Candid Camera,’ and that part had been waiting ever since. Demus’s appearance had only deepened my sense of the surreal and the hope that I might just wake up soon.

The thing about cancer, and I guess any disaster, is that it doesn’t just go away. You don’t wake up. And, in the end, you just have to get on with things exactly like everyone else does. Demus’s appearance was the same, a strange fact I had to bend my life around.