Aging cut man and boxing coach Frankie Dunn owns the Hit Pit, a downtown Los Angeles boxing gym that he runs with the help of his friend, retired boxer Eddie Scrap-Iron Dupris. While Frankie coaches rising boxing talent, Big Willie Little, at the gym, amateurs train, including mildly retarded Texan Danger Barch, bully Shawrelle Berry and the eager Maggie Fitzgerald, the only woman in the gym. When Maggie begs Frankie to coach her, he coldly states that he does not train girls. A stern, tormented man, Frankie attends daily mass everyday to pray for reconciliation with his estranged daughter Katie, and gleefully challenges Father Horvak mercilessly on the fine points of Catholic theology. Although he writes Katie weekly, his letters are always returned unopened. Frankie finds solace in learning Gaelic so he can read Irish romantic poet William Butler Yeats. Meanwhile, Maggie, "trailer trash" from Southwest Missouri, works as a waitress, saving every penny and eating table scraps to pay for six months of gym dues in advance. One night when Scrap finds Maggie diligently working the bag after closing time, he offers her Frankie’s old speed bag and a few tips. Learning that Scrap lives at the gym, Maggie asks to stay late to work and Scrap agrees, knowing that Maggie is a natural. When Frankie finds Maggie with the speed bag, he angrily tells her to quit, assessing that at thirty-one she is too old for training, which would take at least three more years. That night, Willie tells Frank he is leaving him for a new manager, Mickey Mack, who has promised him a title fight, thus cutting Frankie out of any potential winnings. Later at the gym, Scrap is sympathetic but reminds Frank that his need to "protect" Willie from the championship, by refusing him the opportunity for several months, forced the capable fighter to move on. After Willie wins the title championship, Frankie reluctantly takes on Maggie, on the condition that she never question him, and when the time comes, she choose another manager to arrange fights. Maggie agrees, but in her excitement cannot stop asking questions. She explains that with an incarcerated brother, an obese, mean-spirited mother and a sister cheating the government welfare system, she has only boxing, the one thing she feels good about doing. In the ensuing months, Maggie thrives on Frankie's coaching, increasing her agility and confidence, and listens as Frankie constantly warns her to protect herself. Maggie keeps asking Frankie to arrange a fight for her but, still worried that she will be hurt, he callously pushes her away by telling her to go with manager Sally Mendoza. During Maggie's first fight, Scrap tells Frankie that Sally is deliberately setting up Maggie to lose. Unable to sit idly by while she is beaten, Frankie announces that Maggie is his fighter, helps her win and promises never to leave her again. After Maggie easily wins several four-round and six-round fights in the first round, Frankie has to move her up a class, but in the first brutal round of her next bout, the other fighter breaks Maggie's nose. Knowing that the match will be called if the blood is not stopped, Maggie insists that Frankie reset her nose, a trick for which Frankie is famous. With his warning that she has only twenty seconds left before the blood flow will resume, Maggie quickly knocks out her opponent and goes on to win twelve straight fights with early knockouts. When Frankie refuses to let Maggie fight for a championship, Scrap tries to explain Frankie's reluctance by relating that Frankie had patched up Scrap's eye repeatedly during his last fight, and despite Frankie's warnings that he should forfeit the fight, Scrap insisted on staying in the ring and lost his eye as a consequence. Blaming himself for Scrap's partial blindness, Frankie is unable to put his fighters in situations in which they cannot protect themselves. Scrap then sets up a meeting for Maggie with Mickey Mack, but Maggie flatly refuses to leave Frankie. Going to Maggie's run-down apartment one day, Frankie advises her to save her winnings to buy a house. Seeing Maggie's devotion and sacrifice, Frankie agrees to let her compete against the English middleweight champion, a match worth a million dollars. At the match in London, Maggie enters the arena wearing her new silk jacket, a gift from Frankie, embroidered with the Gaelic word "Mo Cuishle," causing the crowd to start chanting the words. At first getting a pummeling by her younger, stronger opponent, Maggie wins the match after Frankie encourages her to go after the woman with her knockout punch. Following a series of successful fights in foreign cities, Maggie returns to the United States. While in Missouri for a match, Maggie asks Frankie to take her to her mother's trailer to surprise her with the title to the house that she has just bought for her family. Maggie is excited and happy to make the gift, but when they arrive, her mother Earline can only complain that the house will endanger her welfare payments, while her sister Mardell laughs at Maggie's career as a fighter. Handing her mother the keys to the house, Maggie suggests she sell it and leaves her family. During their drive home, Maggie tells Frankie a story about her deceased father, who, out of love, killed his dog Axel when the animal could no longer walk because of hip problems. When Maggie then tells Frankie he is all she has left, Frankie tells her that she has him. Knowing that Frankie loves homemade lemon pie, Maggie takes him to a roadside diner she and her father loved, and Frankie says that someday he might enjoy settling down with a place like it. Soon after, Frankie finally sets up a million dollar title match for Maggie with WBA welter weight Billy "Blue Bear,” who is known for being the “dirtiest” fighter in the ring. Although her many fans once again chant "Mo Cuishle" upon Maggie's arrival in the arena, Frankie teasingly refuses to tell her the meaning. During the first and second rounds, Maggie takes a beating and suffers from Blue Bear's elbowing and illegal hits. After taking Frankie's direction to fight back in kind, Maggie knocks Blue Bear down in the third round. Infuriated by the humiliation, Blue Bear rises after the bell and, as Maggie's back is turned, hits her from behind, causing her to fall and hit her head on the overturned corner stool. The tragic incident shatters Maggie's spinal cord, leaving her unconscious for days and dependent on a respirator to live. Frankie holds vigil at the hospital, unable to forgive himself or Scrap for allowing Maggie to fight. Once she regains consciousness, Maggie blames herself for failing to "protect herself" during the fight. Desperate to change the bleak prognosis, Frankie calls dozens of hospitals to try to get medical attention to correct the injury, but there is no hope. As the weeks progress, Maggie's immobility causes skin ulcers, and despite the best care, she is completely paralyzed below the neck. While Frankie arranges for Maggie to be taken to a Los Angeles rehabilitation center and visits her daily, her family does not visit until they have a lawyer to coerce her into signing over all of her assets. Frankie tries to defend Maggie, but she orders him from the room, then lets the pen drop from her mouth and insults her mother for her selfishness until her family leaves the room in humiliation. A short time later, Maggie's leg is amputated because of a gangrenous ulcer, and Frankie talks about getting her a powered wheelchair and entering her in classes at a local college. However, instead of responding to Frankie's enthusiasm, Maggie asks him to do for her what her father once did for Axel, begging him to help her die with the memory of people chanting her name still fresh in her consciousness. Frankie is horrified and refuses, but that night is summoned back to the hospital in the middle of the night because Maggie has bitten through her tongue and nearly bled to death. Maggie continues to bite her tongue, even after repeated attempts to stitch it, until the nurses are forced to sedate her. Visiting Father Horvak, Frankie debates the sin of helping Maggie with this final act, but Horvak advises him to leave her "with God" and tells the emotionally exhausted Frankie that he will be lost forever if he does this thing. Frankie then prepares an injection at the gym, where Scrap, knowing Frankie's plan, tells him that Maggie came to the gym with nothing but a dream, which Frankie helped her fulfill. Late that night, Frankie goes into Maggie's room and, just before he removes the respirator and gives her the fatal injection, reveals the meaning of "Mo Cuishle," my darling, my blood, then kisses Maggie as she peacefully dies. Weeks later, Scrap, who has taken over the gym after Frankie left without a word, finishes a letter to Katie, writing that he thinks she should know the kind of man her father was. In Missouri, Frankie sits in the diner he and Maggie visited and savors a piece of lemon meringue pie.