The sequel opens after the couple's marriage, when a mysterious scent and a string of threatening, inexplicable incidents begin to unsettle the newlyweds and their circle. The young reporter Joseph Rouletabille pursues clues through locked rooms, assaults on a coastal fort, and baffling discoveries including a seemingly impossible corpse. With allies and unexpected arrivals, he organizes defenses, probes motives, and confronts concealed identities until a final confrontation untangles the layered deception and danger surrounding Mathilde and her husband.