The marquis Galeazzo di Torre Alta is murdered by a mysterious killer who calls himself Diabolicus. His heirs are his three brothers and a sister, but all of them, with the exception of Monsignor Antonino di Torre Alta, are killed by Diabolicus. The police are unable to solve the case, but when Antonino gives his inheritance to Pasquale Bonocore, illegitimate son of his father and Pasquale is in prison and therefore can’t be the killer, all the clues are in the hand of the police.

The fake private detective Mike Spillone is hired by two old ladies to find out if Brigitte, the wife of their nephew Otello Bellomo, has a lover. Brigitte is a physician but the two aunts are unaware of the fact. While investigating, Mike and his assistant Johnny discover Brigitte with a prospective patient, the marquis De Vitti who was shot by the husband of the woman he tried to seduce. Afterwards Spillone finds her with her husband who he believes to be her lover.