In Cornwall in 1910, Tithe Hall is sealed against the apocalypse its Viscount expects to accompany Halley's Comet. When he is found murdered in an apparently impossible locked room, suspicion turns to Stephen Pike, a new under-butler rebuilding his life after wrongful imprisonment. Stephen joins forces with Decima Stockingham, the Viscount's unconventional eighty-year-old aunt, to investigate the household. Their search through secret passages, family resentments and class tensions must expose the killer before fear and suspicion claim another victim.