Don Matteo is a thoroughly ordinary Catholic priest with an extraordinary ability to read people and solve crimes. He’s a parish priest who never met an unjustly accused person he didn’t want to help.

Tony is a young Neapolitan musician. He has a lot of problems and a past with drugs, but also with women. He is engaged to Susy, but he betrays her with a university professor, who has an invalid husband, but also with other young aspiring singers, who try in every way to break into the world of music. Among these there is Elena, engaged in various difficulties to organize a concert with the best names of Neapolitan music.

Luigi, a middle-aged engineer who, getting increasingly worried about the rising crime rate in his surroundings, follows the advice and example of his sleazy drinking cronies to arm himself with the titular weapon; the problem is that he starts carrying – and, occasionally, brandishing – it with him everywhere.

A shoot out after a robbery ends with the death of the Chief of Police. Police detective Ghini goes in search of a suspect, Lanza, to avenge the death of his boss. Ghini's search leads him to Belli, a Sicilian gangster and 'Trashy', the dirty leader of a gang of killers. The action is violent as both police and criminals track down Belli who still finds some time to attempt a couple of robberies...