The night watchman is frightened by a terrible ghost - the tragically drowned daughter of a rich merchant. In the morning, the guard has fallen to death from a high tower and several more people lose their lives in suspicious accidents. Investigating these events, Melchior's paths cross with a prostitute, a priest, a foreign artist and a mad heretic. The clues take him to the Pirita Convent, which gloomy vaults hide a dangerous secret buried in oblivion. At the same time, Melchior is troubled by the question of why his beloved Keterlyn suddenly breaks their engagement.