‘OK, so you start off with the world as one big tree.’

Masamune drew a large trunk, and labelled it The World.

‘And our worlds now branch off.’

He drew branches coming out of either side of the trunk. Seven in all.

‘Each of these is one of our worlds – the world I’m in, in Minami Tokyo City, the world Rion’s in, the one Ureshino’s in. And if there’re too many of these worlds, it’s better that some disappear.’

‘But why?’ Ureshino and Kokoro said in unison. Disappear – vanish – neither one sounded good.

‘If it disappears, then what happens to the people in it? Do they die?’

‘It might be a little different from – dying. But they do disappear. It’s as if they were never there to begin with.’

‘But who decides if one is going to disappear? Whose choice is it?’

‘It’s different, depending on the set-up of the novel or manga. Most of the time it’s like the will of the world. Or the will of God.’

Masamune pointed his pencil at the drawing.

‘The trunk part decides that the branches are too heavy, and needs to get rid of some. Most of the time they use the term weed out. It’s like in nature, where those who adapt to the environment survive, and the others die out.’

Masamune looked up. ‘Anyhow, most fictional parallel worlds have that set-up, with worlds being weeded out and selected. Gate W’s like that too, no?’

‘Gate what?’

‘You know it, don’t you? I mean Gateworld. Nagahisa’s mega bestselling game at the moment? You seriously haven’t heard of it? No way.’

‘Nagahisa …?’ Subaru asked dubiously. Masamune looked irritated.

‘Laughlin Nagahisa, guys! The genius director of that video game development company.’ As he spoke, he seemed to give up. ‘Jeez – do I have to start from scratch?’ He scratched his head. ‘Do you actually need me to explain the famous parallel world story? You people don’t even know the most basic things.’

‘I know it. They made it into a movie too, didn’t they?’ Ureshino said.

‘No, they didn’t. Forget it. If you don’t know, then just keep quiet.’ Masamune shook his head as if they were getting nowhere.

‘In Gateworld,’ he went on, ‘the representatives from each of the parallel worlds get together to fight it out. The ones who lose, their worlds vanish, so it’s a story about which world survives. The winning world becomes the sole trunk left. So they all fight to the death so their world can continue. That’s the kind of game it is.’

‘Are you saying that’s what’s happening to us?’ Subaru asked.