After terrible epidemics fifteen years earlier, Britain has been left in "post-war" poverty. The country is run by the Purity trying to eradicate "S.I.D", a retro-virus that turns people into monsters that die in agony and insane.
Chloe Dark, 16, wants to make life better, defeats the school bully using Jujitsu. She works with Miss Kepple, the school Purity teacher, so she can join the Purity herself. Then her friend Melinda disappears, like two other teenagers before her.
People around Chloe tell her she can't do anything; she should leave it to the Police and the Purity. But she can't, though she doesn't know what to do. The Police interview Chloe and she admits Melinda was pre-occupied: if only she'd realised!
Fighting the bully inflamed Chloe's back trouble; she goes to her chiropractor, Ali. He scans her back but, after she's gone, discovers she's growing wings: she's an S.I.D infectee. He's scared; he should report her but the Purity kills freaks publicly.
Ali deletes the scan in panic but it's too late. Someone monitoring the medical network has noticed and knows Chloe is a freak. Her kidnap is ordered, and Ali's death.
Ali decides to tell Chloe's parents but when he meets them her father is violent and won't believe him. On his way home Ali is run down by a lorry and killed. Meanwhile Chloe is attacked by the men who took Melinda; she manages to escape, but only just.