At the Hialeah racetrack in sunny Miami, Florida, we find the once-renowned boxing champion Billy "The Champ" Flynn, who now embraces a different vocation as a horse trainer, living above the stables with his beloved eight-year-old son, T. J.. After a lengthy seven-year hiatus from the boxing scene, Billy contemplates making a return to the ring. He takes T. J. along for a visit to the Ninth Street Gym, where he hopes to connect with his manager, Charlie Goodman. T. J., who idolizes his father and affectionately calls him “Champ,” is ecstatic at the prospect of Billy possibly stepping back into the boxing limelight.
When they arrive at the gym, Billy is met with nostalgia, as Georgie, a training assistant, has preserved his old gloves. Unfortunately, Billy’s excitement is dampened when he feels slighted by Goodman's tardiness and storms out, leaving T. J. at the gym. The young boy later finds his father inebriated at a bar and successfully aids him in making it back home. In a moment of desperation, Billy pilfers $20 from T. J.'s piggy bank, igniting a reckless night at a casino. To everyone's surprise, he returns the next morning with a fortune of $6,400 and gifts for his surroundings. Just when T. J. fears being overlooked, Billy surprises him with a thoroughbred racehorse named "She's a Lady." Thrilled, T. J. becomes the talk of the stable, joyfully riding around on his new filly.
The excitement reaches its peak on race day at Hialeah, where T. J. prepares his horse and jockey for what he believes will be a triumphant run. Among the spectators is Annie Phillips, a successful fashion designer and member of the elite society, who overhears T. J.'s confident predictions about "She's a Lady." Enchanted by his enthusiasm, Annie agrees to place a $10 bet on T. J.'s horse. During the race, just as the filly appears poised for victory, she suddenly falls.
From the stands, Annie, through her binoculars, recognizes her ex-husband, Billy, following T. J. as the boy races across the track to comfort his fallen horse. It dawns on Annie that T. J. is the child she had left behind seven years ago. Meanwhile, at the stables, the pair anxiously await the veterinarian's verdict on the fate of "She's a Lady." When Billy spots Annie approaching, he hurriedly pulls her aside to reveal that he has kept her absence a secret from T. J. by fabricating a story about her tragic death in a car accident.
Though Annie acknowledges her past mistakes, her intention to reconnect with her son is unwavering. Just as they argue, T. J. bursts in with the joyous news that the horse’s injury is merely superficial. Annie reintroduces herself, but Billy swiftly takes T. J. away, ill-prepared for the confrontation. After some soul-searching, Billy permits his son a visit with Annie, concealing the fact that she is his biological mother. T. J. enjoys a splendid day aboard Annie's yacht, excitedly sharing that she and her husband, Mike, are kind and not at all pretentious like the typical "turf clubbers."
However, trouble looms when Billy’s luck shifts at the casino, leaving him with a $2,000 debt to Whitey, a backer who threatens to claim "She's a Lady" if not settled within forty-eight hours. In a moment of desperation, Billy turns to Annie for help. When he cannot meet Whitey’s demands, he becomes enraged, physically confronting him at the stables, resulting in a disastrous fight that leads to his arrest. In jail, he learns that Annie wishes to look after T. J. while the situation is resolved. Conflicted and stubborn, Billy tries to convince T. J. that he is a failure and lashes out, claiming he’s done with being a father, only to break down shortly after.
On the yacht, during an innocent unpacking session with T. J., Annie accidently reveals her identity, prompting confusion and distress for the young boy who insists his mother has died. After being released from jail, Billy goes to find T. J. waiting at the racetrack with his suitcase. They share a heartwarming embrace, with Billy vowing never to leave again.
Later that evening, Annie visits the stables to discuss T. J.'s future with Billy, who is struggling with his own emotions. It becomes apparent to Annie that Billy's feelings for her are still alive. Eager to provide a better life for his son, Billy resolves to stage a boxing comeback. His former trainer, Jackie, arrives but expresses concerns about Billy's health due to past head injuries. Dismissing the warnings, Billy insists he wants to train and prepares for a comeback bout. He encourages T. J. to open up to Annie through letters that she has been sending.
In New York, Annie confides in her husband about her desire to support Billy’s fight, believing her presence could make a pivotal difference. Excitement fills the air as the night of the match arrives at Miami's Forum, with T. J. and Billy anxiously scanning the audience for Annie. As the fight begins, Billy starts strong, knocking down his opponent, Bowers. However, as he sustains a cut near his eye, the tide turns, leaving him vulnerable. Despite experiencing disorientation and being on the verge of defeat, Billy refuses to be stopped.
Summoning his strength, he delivers a final blow, knocking Bowers out and securing victory. T.J. beams with pride, but concern quickly fills his face upon seeing his father’s battered appearance. While leaning against Jackie, exhaustion overtakes Billy, and he collapses backstage. In his final moments, as he lies on the table, he advises T.J. that Annie is a good person and to hold onto the happiness that comes from his victory. Tragically, as T. J. cries out for "The Champ," he is met with the harsh reality that his father has departed. When Annie enters the room, she instinctively rushes to comfort her son, sharing in the profound loss that has just occurred.
