Just out of prison, ex-con Ugo Piazza meets his former employer, a psychopathic gangster Rocco who enjoys sick violence and torture. Both the gangsters and the police believe Ugo has hidden $300,000 that should have gone to an American drug syndicate boss. Caliber 9 is the first part in Di Leo's "Milieu" Trilogy of poliziotteschi films. It was followed by "The Italian Connection" (1972) and "The Boss" (1973).