At the Hialeah racetrack near Miami, Florida, former boxing champion Billy “The Champ” Flynn works as a horse trainer and lives in an apartment above the stables with his eight year-old son, T. J. Considering a comeback after seven years away from the sport, Billy returns to the Ninth Street Gym for a meeting with manager Charlie Goodman and brings along T. J., who is thrilled that his father, whom he idolizes and calls “Champ,” might fight again. At the gym, Billy is still revered as a boxer, and Georgie, a training assistant, has saved Billy’s old gloves. However, Billy feels disrespected when Goodman keeps him waiting, and shouts that he will return in thirty minutes, leaving his son at the gym. Later, T. J. finds his father drunk at a bar and helps him home. In the middle of the night, Billy steals $20 dollars from T. J.’s piggy bank and drives to a casino. Returning the next morning, he announces to the stable employees and T. J. that he won $6,400 and bought gifts for everyone. Just as T. J. thinks his father has forgotten him, Billy presents him with a racehorse, named “She’s a Lady.” T. J. becomes the center of attention as he rides around the stable, the proud owner of a filly. On race day at Hialeah, T. J. prepares his horse and jockey for the next heat. Nearby, Annie Phillips, a rich, successful fashion designer who is part of the society crowd in attendance, overhears T. J. bragging that “She’s a Lady” will win. Charmed by the boy's gumption, Annie promises T. J. that she will bet $10 dollars on the horse. During the race, “She’s a Lady” is on the verge of taking the lead when she falls. From the stands, Annie watches through her binoculars as T. J. runs across the track to his stricken horse. When she recognizes her ex-husband, Billy Flynn, following the boy, she realizes that T. J. is the son she left seven years before. At the stables, T. J. and Billy wait anxiously for the veterinarian to decide the fate of “She’s a Lady.” As soon as Billy sees Annie approaching, he yanks her aside to disclose that he explained her seven-year absence by telling T. J his mother had been killed in a car accident. Annie understands that she gave up custody during the divorce, but wants to see her son. T. J. interrupts their argument with the good news that his horse’s wound is only superficial. Annie reintroduces herself to T. J., but Billy takes his son and walks away. After reconsidering, Billy allows T. J. to visit Annie one afternoon, without revealing that Annie is his mother. Dressed in a suit, T. J. spends the day on the Phillips’ yacht. Afterward, he is enthusiastic about the visit, telling his father that Annie and her husband, Mike, are not snobby like most “turf clubbers.” Sometime later at a casino, Billy’s winning streak comes to an end and he incurs a debt of $2,000 dollars, which he must pay in forty-eight hours or the backer, Whitey, will take possession of “She’s a Lady.” After unsuccessfully pursuing other options, Billy borrows the money from Annie. Later, he is furious when Whitey arrives at the stables to collect T. J.’s horse, since Billy assured him that he would deliver the $2,000 dollars that afternoon. Billy pulls Whitey from the car and punches him. When security guards try to intervene, Billy attacks them until police arrive and arrest him. In jail, an attorney informs Billy that Annie wants to look after T. J. while the case is settled. Billy tells his son about the arrangement, but T. J. does not understand. To convince his devoted son that he is “a bum,” Billy declares that he is tired of taking care of T. J. and slaps him. On the yacht, Annie helps T. J. unpack when she accidently reveals that she is his mother. As she tries to explain, T. J. says that his mother is dead and screams for her to go away. After being released from jail, Billy returns to the racetrack where he sees T. J. in the stands with his suitcase. They hug, and Billy says that he will never leave again. That evening, Annie arrives at the stables to confer with Billy about T. J., since the boy will need an explanation now that he knows his mother is alive. As Billy becomes emotional about his own needs, Annie realizes that Billy still loves her. Motivated to start earning a decent living for his son, Billy commits to a boxing comeback. His former trainer, Jackie, arrives in Miami, but expresses concern about Billy’s previous blows to the head and recommends a medical evaluation. However, Billy dismisses his headaches and says he has made up his mind. Jackie agrees to help, and they begin training. Meanwhile, Billy encourages T. J. to respond to his mother’s letters. At her residence in New York City, Annie explains to her husband that she wants to be in Miami for Billy’s fight, sensing that her presence will make a difference. On the night of the match at Miami’s Forum, T. J. and Billy are excited to see Annie in the crowd. During an early round, Billy looks strong and knocks his opponent, Bowers, to the floor. In the next round, Billy receives a cut near his eye, but fights back. As the match continues, Billy is briefly knocked down and staggers to his feet, experiencing double vision. When he goes down a second time, the referee considers stopping the fight as Billy’s cut continues to bleed. Jackie and T. J. also want to end the fight, but Billy assures them he is okay. All of a sudden, Billy finds a second wind and knocks out Bowers for the victory. He holds up his son as the crowd cheers, but T. J. is alarmed by his father’s disorientated and swollen appearance. While leaning against Jackie on the way back to the locker room, Billy collapses. He lies on the table and tells his son that Annie is a good person and to be happy because he won the fight. As Billy dies, T. J. calls for “The Champ” and asks the other trainers to wake him, but Jackie announces Billy is gone. When she enters the room, Annie goes to her son and hugs him.