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 except 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.