In Paris, jewel thief Roger Sartet escapes from custody with the help of the Manalese, a small-time but well-organised Sicilian Mafia clan led by patriarch Vittorio and which includes his sons Aldo and Sergio, and son-in-law Luigi. While in prison, Sartet got to know an electrician who was involved in the setting up of an extensive security system at a diamond exhibition in Rome — the electrician returned home early, unannounced, caught his wife in bed with a lover and shot them. Unaccustomed to prison life, he made friends with Sartet and bit-by-bit supplied him with details of the exhibition.