In 1941, Jake LaMotta is a young, up-and-coming middleweight boxer who suffers his first loss to Jimmy Reeves after a controversial decision. Jake's brother Joey discusses a potential shot for the middleweight title with one of his Mafia connections, Salvy Batts, but he repeatedly refuses the Mafia's help, wanting to win the championship on his own terms. Some time after, Jake spots a fifteen-year-old girl named Vickie at a swimming pool in his Bronx neighborhood. He eventually pursues a relationship with her, although he is already married and Vickie is underage. In 1943, Jake defeats Sugar Ray Robinson, and has a rematch three weeks later. Despite Jake dominating Robinson during the bout, the judges surprisingly rule in favor of Robinson, who Joey feels won only because he was enlisting into the Army the following week. By 1945, Jake marries Vickie.
Jake is controlling and domineering over Vickie, and constantly worries that she has feelings for other men. His jealousy is evident when he brutally beats his next opponent, Tony Janiro, in front of Tommy Como, the local mob boss, and Vickie. As Joey discusses the victory with journalists at the Copacabana, he is distracted by seeing Vickie approach a table with Salvy and his crew. Joey speaks with Vickie, who implies she is dissatisfied in her marriage with Jake. Under the wrong impression that Vickie is having an affair with Salvy, Joey viciously attacks him in a fight that spills outside of the club. Como later orders them to apologize, and has Joey tell Jake that if he wants a chance at the championship title, which Como controls, he will have to take a dive first. Jake purposely loses his next match against Billy Fox, and is booed out of the building after putting up a lackluster performance. He is suspended shortly thereafter from the board on suspicion of throwing the fight, much to his anguish. He is eventually reinstated, and in 1949, wins the middleweight championship title against Marcel Cerdan.
A year later, Jake becomes increasingly paranoid that Vickie is having an affair. He asks Joey if he has had an affair with her, enraging Joey and causing him to leave. Jake later presses Vickie about whether she has had an affair, leading to her sarcastically confessing that she had sex with Joey, Salvy and Tommy. In a fit of rage, Jake, followed by Vickie, walks to Joey's house and assaults him in front of his wife Lenora and their children before knocking Vickie unconscious. Vickie later returns to their home and threatens to leave, but they reconcile. After defending his championship belt in a grueling fifteen-round bout against Laurent Dauthuille in 1950, he calls his brother after the fight to make amends, but when Joey assumes Salvy is on the other end and starts insulting and cursing at him, Jake silently hangs up. Estranged from his brother, Jake sees his career decline and he eventually loses his title to Sugar Ray Robinson in their final encounter in 1951.
By 1956, an aged and overweight Jake has retired and moved with his family to Miami. After he stays out all night at the nightclub which he owns, Vickie tells him she wants a divorce as well as full custody of their children. She also threatens to call the police if he comes anywhere near them. He is later arrested for introducing under-age girls to men in his club. He unsuccessfully attempts to bribe his way out of his criminal case using the jewels from his championship belt. In 1957, he goes to jail, sorrowfully questioning his misfortune and crying in despair. Upon returning to New York City in 1958, he encounters Joey, who reluctantly accepts his embrace.
In 1964, Jake performs stand-up comedy at various clubs. Backstage before a show LaMotta prepares himself for his performance by shadowboxing, quoting scenes from On the Waterfront and chanting "I'm the boss" before taking the stage.
