Maggie Fitzgerald, a determined waitress from the Ozarks, dreams of becoming a professional boxer. She walks into the Hit Pit, a dilapidated boxing gym in Los Angeles run by Frankie Dunn, a grumpy and emotionally guarded trainer who is estranged from his daughter. Maggie approaches Frankie, requesting him to train her, but he initially refuses, citing his disapproval of training women and believing she is too old to start a boxing career. Despite his resistance, Eddie "Scrap-Iron" Dupris, Frankie’s loyal friend and the gym’s handyman, allows Maggie to train at the gym, seeing her potential.
As Maggie fights her way through the amateur ranks, her talent for quick knockouts begins to shine, attracting attention. Frankie’s pride and concern lead him to bribe managers to keep her safe from overly aggressive or unscrupulous opponents who might exploit her. Meanwhile, her sparring partner and Frankie’s prize prospect, "Big Willie" Little, grows impatient with Frankie’s hesitation to endorse a championship fight, ultimately signing with a rival manager Mickey Mack. Despite these setbacks, Maggie's persistence pays off, and she earns a shot at the big leagues.
With Frankie’s coaching, Maggie excels in the women's boxing division, gaining a reputation for her powerful punches and resilience. Her determination is so fierce that Frankie bestows upon her a Gaelic nickname, Mo Cuishle, embroidered on her boxing robe. He keeps its meaning a secret from her, adding a layer of mystique to her journey. Maggie continues to win fights across Europe, gradually building her reputation and financial stability, even saving enough money to purchase her mother a house. However, her mother, resentful and concerned, criticizes her for risking her safety, accusing her of being a laughingstock back home for pursuing such a dangerous sport.
Eventually, Frankie agrees to arrange a title fight for Maggie. The big moment arrives in Las Vegas, where she faces Billie "The Blue Bear" Osterman, a fierce German ex-prostitute known for her dirty fighting tactics. The fight begins with Maggie dominating, but Billie, desperate and unscrupulous, throws an illegal punch after the bell rings to end the round, knocking Maggie out. The blow results in Maggie falling hard onto her corner stool, breaking her neck and leaving her a quadriplegic dependent on a ventilator.
Following the injury, Frankie plunges into a deep emotional crisis, moving through the five stages of grief. He seeks numerous medical opinions, battling denial, eventually lashing out in anger at Scrap, and desperately trying to bargain with a higher power in prayer. Over time, he recognizes his guilt and feels responsible for Maggie’s devastating injury. Scrap reassures Frankie that Maggie’s fight was her choice and her victory—an achievement she owed to Frankie’s mentorship.
While hospitalized, Maggie looks forward to her family’s visit. Their arrival is delayed, as they first spend time at Disneyland and Hollywood attractions, only to reveal a selfish concern: they are primarily there to secure her assets through a lawyer. Disgusted by their greed, Maggie orders them to leave and threatens to expose their welfare fraud if they contact her again.
As her condition deteriorates, Maggie develops bedsores and faces the amputation of her infected leg. She confesses to Frankie her wish to end her life, feeling she has achieved all she wanted. Frankie, overwhelmed and horrified, refuses to help her die. Later, in a desperate act, Maggie bites her tongue, trying to bleed to death. Frankie’s priest warns him that if he goes through with Maggie’s request, he will lose a part of himself forever. Ultimately, Frankie secretly visits her in the hospital, where, just before administering a fatal dose of adrenaline, he shares the meaning of her Gaelic nickname—Mo Cuishle, which signifies my darling, and my blood. They share a heartfelt final moment, and Frankie gives Maggie a compassionate farewell kiss before leaving, retiring from boxing entirely.
Scrap later writes to Frankie’s daughter, revealing the depth of Frankie’s true character—his capacity for love, sacrifice, and unwavering loyalty—lessons that transcend the boxing ring.
