It is the Roman holiday of Saturnalia. Marcus Didius Falco and Helena have returned from Greece only to find that Helena's brother Justinus's marital problems have exploded. Justinus's first love, Veleda, a tribal leader and prophetess from Germania, has been brought to Rome and put under house arrest pending a ritual sacrifice at her capturer's Triumph. Justinus is love-struck once more and his wife, the temperamental Claudia, is enraged. Then Veleda escapes leaving behind a corpse. Justinus disappears too and it is up to Falco and the Chief Spy Anacrites to try to find the missing couple all against the backdrop of the orgiastic holiday period when literally anything goes . . .