The plot's central tragedy is driven by the escalating rivalry between Rocco and Simone over the prostitute Nadia. After Rocco begins a tender relationship with Nadia during his military service, Simone's jealousy explodes. In the film's most infamous scene, Simone, with the help of his friends, corners Rocco and Nadia. He brutally rapes Nadia while forcing a helpless Rocco to watch, irrevocably shattering the bond between the brothers and setting the stage for the final tragedy.
Despite this horrific act, Rocco's commitment to family loyalty leads him to sacrifice his love for Nadia, encouraging her to return to Simone in a misguided attempt to save his brother. This only pushes Simone further into despair and debt. To pay off Simone's debts, Rocco signs a long-term, restrictive boxing contract, effectively selling his future.
The film's climax is a devastating intercutting of two events. While Rocco is in the ring, fighting and winning a championship bout that he hopes will save the family, Simone confronts Nadia one last time. When she tells him how much she hates him for ruining the one good thing in her life, he viciously stabs her to death near the Ponte della Ghisolfa. Simone returns to the family's celebration, bloody and broken, and confesses to the murder. Rocco, in his ultimate act of flawed forgiveness, still wants to protect him. However, Ciro, representing the new world's morality, refuses and leaves to call the police, sealing the family's tragic fate and signaling the final disintegration of their old-world values.