Diana Guzman is a Brooklyn teenager whose hot temper gets her into trouble at school as she repeatedly starts fights with other students. Her frustration stems from her unhappy home life; she lives in a public housing estate with her brother Tiny and their single father, Sandro. Sandro pays for Tiny’s boxing training in hopes of his becoming a professional boxer, although Tiny would prefer to be an artist.

After visiting Tiny’s gym and intervening in a spar to defend him, Diana asks the trainers to let her box, too. She is told she can train there, but not compete in actual fights. When she learns that she cannot afford coaching from Tiny’s trainer, Hector Soto, she asks her father for an allowance but he tells her to get a job. She resorts to stealing his money instead and returns to the gym, where Hector begins to teach her the basics of boxing.

Diana’s first spar is with Adrian Sturges, whom she later meets again when Hector takes her to a professional fight. Adrian invites Diana to dinner after the fight and kisses her after walking her home. One night after a spar which gave Diana a black eye, Sandro sees Diana and Adrian together and confronts her, assuming that she is in an abusive relationship. She storms out of the apartment and spends the night with Adrian. When he asks about her parents, she reveals that her mother committed suicide several years ago. When Diana returns to her apartment, Tiny offers to give up boxing so that she can use the coaching money he gets from their father.

Diana later goes to Hector’s birthday party, but leaves when she sees Adrian getting friendly with his ex-girlfriend. When Diana and Adrian spar at their next session in the gym, he is reluctant to hit her, and she leaves before he can talk to her. Diana’s first amateur match is scheduled against another girl, but when her opponent pulls out she ends up fighting a man, Ray Cortez. Sandro arrives in the middle of the fight to see the match end in Ray’s disqualification for illegal shoving. When Diana arrives home, Sandro berates her for looking like a loser. She retaliates by beating him to the floor and accuses him of abusing her mother to the point of suicide.

After weeks of rigorous training, Diana wins another amateur fight, this time against a girl, Ricki Stiles. Although Diana has accepted Adrian’s apology, tensions rise between them again when they learn that they both have advanced to the finals in their division to fight each other. Adrian refuses to fight a girl and Diana struggles to convince him to view her as a legitimate opponent. He turns up for the fight on the day, however, and after an even match, Diana wins with a unanimous decision by the judges. After the fight, Adrian fears that he has lost Diana’s respect, but she tells him she respects him even more for fighting her, and they reconcile.