Leah Jackson - in detention. Then armed Year 9s burst in, shooting. She escapes, just. But the new Lock Down system for keeping intruders out is now locking everyone in.

She takes to the ceilings and air vents with another student, Anton, and manages to use her mobile to call out to the world.

First: survive the gang - the so-called 'Eternal Knights'.

Second: rescue other kids taken hostage, and one urgently needing medical help. Outside, parents gather, the army want intelligence, television cameras roll, psychologists give opinions, sociologists rationalise, doctors advise - and they all want a piece of Leah.

Soon her phone battery is running out; the SAS want her to reconnoitre the hostage area ...

But she is guarding a terrifying conviction. Her brother, Connor, is at the centre of this horror.

Is he with the Eternal Knights or just a pawn? She remembers. All those times Connor reached out for help ... If she'd listened, voiced her fears about him earlier, would things be different now?

Should she give up her brother?With only Anton for company, surviving by wits alone, Leah wrestles with the terrible choices ...