Umar was added in at a fairly late stage. Initially the character who helps Frank at the end was white, but I realised the book wasn’t exactly representative of the typical make-up of a British school. It’s hard to write a character from a different ethnic group from your own. I worried about getting it wrong, and ending up with what would feel like a ‘token’ ethnic character.

Umar is a blend of a type of student I’ve met: with super high achieving parents – often doctors. His mum doesn’t speak English, so he shares Frank’s experience of being put under pressure, straddling two cultures.

The link to terrorism was lucky. Umar gets blamed for what Frank did when he bombed the school with plaecivia, and made it more obvious that what both Frank and Sally are doing are acts of terrorism. The terrorist isn’t Umar. It’s Frank.

‘It was them.’ She pointed at Umar then at Iqra.

‘Don’t be ridiculous,’ I said. ‘There aren’t any terrorists any more.’

‘They’ve got biological weapons, my dad says.’

‘Then why didn’t they make any demands?’

‘Because they want to wipe us out, stupid,’ Georgina said. ‘They don’t want to negotiate. They just want us dead.’

She was pointing straight at Umar. He was grey, smaller than usual.

‘I already told you. It’s nothing to do with me. No one knows who did it.’

‘Everyone knows,’ Georgina said, giving him a meaningful look.

So then I really did feel sick. But there was nothing I could do.

There’s a kind of fanaticism that terrifies me. People who are so convinced that they’re right that they think they can justify anything. Acts of terror in Iraq, Afghanistan, Syria, Sudan, Nigeria are nightmarish reminders that some people feel drawn to destruction – just to make a point. Women are killed, children are killed, men are killed – then more are killed in retribution.

In his battle with Toad, Frank feels the same emotions – the giddying feeling of power.

Streets screamed. People stumbled to get out of the way, crushed bodies underfoot. When kids tried to get on the buses, everyone got off – but they were already contaminated with the dust. I had to walk home in the end, along deserted streets.

It must, I thought, be a lot like the Apocalypse.

I felt frightened. Sick. Powerful.

It’s like a monster that feeds on itself.

The higher you stack it, the worse it gets, but it’s balanced, like you’re obsessed with balance, throwing flame onto fire onto dynamite onto a firework factory on a missile plant on a nuclear reactor. Maybe the only perpetual motion machine on this planet is people getting angry with each other. It feels like being sick, but you can’t. I don’t want to swallow it.

Eris says I don’t have a choice. I got defeated. But then it came to me. I know how to stop Toad. It’s perfect and it terrifies me.

There’s going to be a crater from this idea fifty miles wide.