La supertestimone is a 1971 black comedy Italian film, directed by Franco Giraldi.

One Friday evening, the girl is found killed: Marino, immediately suspected, proves to have an alibi.

From the photos in the newspapers, which devote ample space to the squalid event and to its characters, Isolina Pantò - extravagant owner of a private asylum and former servant in a convent - recognizes the suspect whom she remembers seeing at the crime scene.

Determined to make amends, Isolina lovingly assists the inmate, even moving to nearby the penitentiary in order to be closer to him.

Finally free, Marino - after pretending not to be able to get a job - induces Isolina into prostitution and, when the woman shows that she is too fond of a client, he does not hesitate to reveal to her that he was the one to kill Tiziana.