The discovery of a crucified cat in the chapel of his school plunges headmaster Augustus Braithwaite into a battle against forces, and worlds, he previously didn't know existed.

Augustus acquires an ally in Sharna, a young woman he rescued from execution in her own world, and they return to his school to confront the power head on.

Only one of their enemies, teacher Edward Braine, horribly disfigured but still maliciously strong, survives. Together they track him across Ireland to a final confrontation on the island of Inishbofin.