Set after the fall of the caliphate and in the wake of an unprecedented terrorist attack near the Tower of London, the head of SO15 receives contact from a sleeper agent long thought dead. It is Halloween, the night of a new moon. Is it a trick or a treat? Is Housemartin still active, or is it a hoax? With resources stretched, two new recruits move into the former home of the agent to await his possible return. The pair soon realise that they need to find the link between Housemartin and the mysterious murder of a man on the night of the bombing - and they need to do it before the bombers strike again.