A young commissioner investigates the murder of a girl, which took place the previous night in a tram carriage. In order to solve the case, the commissioner is willing to do anything, even to jeopardize the life of his girlfriend.

The constant sensation of imminent danger and the "black" climate typical of Dario Argento's production: in the last episode, a woman tells of having witnessed a murder, but, having no evidence to provide, she is not believed.