In the high summer of 77AD, Roman informer Marcus Didius Falco is beset by personal problems. Newly bereaved and facing unexpected upheavals in his life, it is a relief for him to consider someone else's misfortunes. A middle-aged couple who supplied statues to his father, Geminus, have disappeared in mysterious circumstances. They had an old feud with a bunch of notorious freedmen, the Claudii, who live rough in the pestilential Pontine Marshes, terrorising the neighbourhood.
When a mutilated corpse turns up near Rome, Falco and his vigiles friend Petronius investigate, even though it means travelling in the dread marshes. But just as they are making progress, the Chief Spy, Anacrites, snatches their case away from them. As his rivalry with Falco escalates, he makes false overtures of friendship, but fails to cover up the fact that the violent Claudii have acquired corrupt protection at the highest level. Making further enquiries after they have been warned off can only be dangerous - but when did that stop Falco and Petronius?
Egged on by the slippery bureaucrats who hate Anacrites, the dogged friends dig deeper while a psychotic killer keeps taking more victims, and the shocking truth creeps closer and closer to home...