Returning from abroad to Sicily, a former diplomat informs the police about a strange discovery he has just made. The next day, police inspector Lepri comes to the man's place and finds him dead. Has he committed suicide? While conducting his investigation, Lepri happens to suspect his own commissioner - Meanwhile, a train comes to a stop somewhere in the countryside because of a malfunctioning signal. A motorist decides to alert the station master but he too is found dead, as well as another railway agent. For the police, the driver cannot but be linked to those mysterious deaths.