Kas takes my fingers and runs them over the back of her left hand. There's something hard under the skin.
'What's that?' I ask.

'It's an implant. All Sileys have them. So we could be tracked - back when there was the technology to do it.'

I wait on the surface to get my breath, then dive. There is a blue down here different from up on the surface - it presses in on you and darkens as you go deeper. The sun filters down in shafts but eventually they're swallowed by the blue too.

'You know how sorry I am for your loss. I lost a daughter too.'

'Sorry, bullshit, Harry! You were lucky. You've got a wife to keep you warm at night. And another daughter.'

'You can't blame me for that,' Harry says.

'I'm not blaming you, but we're coming at this current situation from different angles. The girl's old enough to have children. That's about all Sileys are good for - that and working the fields. If they wanted otherwise they shouldn't've come to this country in the first place.'

Stella cuts in then. 'They were children, Tusker. Children. They had no say in where their parents brought them.'

'More bullshit,' Tusker says. 'Besides, everyone knows they brought the virus.'

'You got family?'

'Had. All dead,' I say. 'Dad went first, then Mum. Got no brothers or sisters.'

'And you've survived how long?'

'I dunno. Maybe two years.'

'That's pretty right. We've been trying to keep track of time, but the seasons aren't what they used to be. Long summers. Wild winters.'

Everything up until now has been like a Boys' Own adventure story I could have read when I was a kid - the clever boy outsmarting the bad guys and saving the girl. But this is real, this is holding a blade at a man's throat and looking him in the face. His eyes are opening and closing and there's blood gurgling in his throat as he tries to breathe. I know I can kill him with just a bit of pressure on the knife, but even though I feel that he doesn't deserve to live I can't bring myself to do it. Maybe it's Dad telling me there's always something good in everyone, or maybe it's something deep inside me that makes the decision.