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.
| Actor | Role |
|---|---|
 TotòGaleazzo / Carlo / Scipione / Antonio / Laudomia / Pasquale | Galeazzo / Carlo / Scipione / Antonio / Laudomia / Pasquale |
 Raimondo VianelloBarone Lallo | Barone Lallo |
 Nadia SandersDonna Fiore | Donna Fiore |
 Luigi PaveseCommissario di polizia | Commissario di polizia |
 Mario CastellaniIspettore Scalarini | Ispettore Scalarini |
 Peppino De MartinoNotaio Cocozza | Notaio Cocozza |
 Giulio MarchettiDetective Privato | Detective Privato |
 Franco GiacobiniDottor Santoro | Dottor Santoro |
 Pietro De VicoIl paziente | Il paziente |
 Mimmo PoliIl postino | Il postino |
 Béatrice AltaribaDiana | Diana |