According to legend there is a curse on the ancient castle of the Desgrèz family. Years before, the mistress of one of their ancestors was accused of witchcraft; and when she was burned at the stake, she cursed her lover and his descendants. Now living at the castle is Mathias Desgrèz, an ailing eccentric who is a student of black magic and sorcery. Visiting the old man are his two bickering nephews, Stéphane and Marc, and the latter's wife, Lucie. Also present are mystery writer Michel Boissard and his wife, Marie. On the night of a costume ball the housekeeper sees a shadowy figure, which she believes to be Lucie, give a drink to Mathias. The next morning the old man is found dead from poisoning, and Lucie becomes the chief suspect since she and Marc are the beneficiaries of Mathias' will. It is then disclosed that Mathias' nurse, Myra, is Marc's mistress and that she has been administering cumulative doses of arsenic to Mathias but was not responsible for the final, lethal drink. The two lovers quarrel violently over the sudden turn of events, and Marc drowns Myra. Though Marc confesses to his crime, the murder of Mathias remains unsolved.