The narrative opens with the private wedding of Robert Darzac and Mathilde Stangerson and soon turns into a new wave of mysteries when an inexplicable perfume and a series of frightening incidents disturb their peace. Reporter-detective Joseph Rouletabille reopens investigations that uncover secret identities, impossible corpses, assaults on coastal fortifications, and the return of a presumed-dead antagonist. The plot alternates claustrophobic scenes of siege and daring discoveries, balancing gothic atmosphere with methodical reasoning, while exploring themes of obsession, the persistence of past crimes, and the limits of rational explanation as hidden connections are progressively revealed.