Micky Ward (Mark Wahlberg) is an Irish-American welterweight boxer from a working class family in Lowell, Massachusetts. Managed by his mother, Alice Ward (Melissa Leo), and trained by his older half-brother, Dicky Eklund (Christian Bale), Micky has not had a particularly successful career: He's become a "stepping stone" for other boxers to defeat on their way up. Complicating matters, Dicky, a former boxer whose peak of success was knocking Sugar Ray Leonard down in an HBO televised match, has fallen apart since his early success, becoming addicted to crack cocaine. He is now being filmed for an HBO documentary he believes to be about his "comeback".



On the night of an undercard fight in Atlantic City, Micky's scheduled opponent is ill, and a substitute is found who is 18 pounds heavier than Micky. Despite Micky's reservations, his mother and brother agree to the terms so that they can all get the purse. Micky is soundly defeated by the much heavier fighter in an obvious mismatch. Frustrated with his career and embarrassed by his defeat, Micky tries to retreat from the world and forms a relationship with Charlene Fleming (Amy Adams), a former college athlete who dropped out and became a bartender.



After several weeks, Alice arranges another fight for Micky, who, concerned it will turn out the same as before, shows serious hesitation. His mother and seven sisters blame Charlene for his lack of motivation. Micky mentions that he's received an offer to be paid to train in Las Vegas, but Dicky says he will match the offer so he can keep training and working with his family. Dicky then tries to get money by posing his girlfriend as a prostitute and then, once she picks up a client, impersonating a police officer to steal the client's car. This is quickly foiled by the actual police and Dicky is arrested after a chase and a fight with them. Micky intervenes to try to stop the police from beating his brother, but a police officer breaks his hand before arresting him. At trial, Micky is released but Dicky is sent to jail. Finally fed up, Micky washes his hands of Dicky.



On the night of the HBO documentary, Dicky's family, and Dicky himself in prison, are horrified to see it is High on Crack Street: Lost Lives in Lowell, a documentary about how crack addiction ruined Dicky's career and life. Devastated, Dicky begins training and trying to get his life together in prison. Micky is lured back into boxing by his father, who believes Alice and his stepson Dicky are bad influences. Gathering the other members of his training team and finding a new manager, Sal Lonano, they convince Micky to return to boxing with the explicit understanding that his mother and brother will no longer be involved. They place Micky in minor fights to help him regain his confidence. He is then offered another major fight against an undefeated up-and-coming boxer. During a prison visit, Dicky advises Micky on how best to work his opponent, but Micky feels his brother is being selfish and trying to restart his own failed career. During the actual match, Micky is nearly overwhelmed at the beginning but, remembering his brother's advice, alters his strategy and is eventually triumphant; he earns the title shot for which his opponent was being groomed.



Upon his release from prison, Dicky and his mother go to see Micky train. Assuming things are as they were, Dicky prepares to spar with his brother, but Micky informs him that he's no longer allowed per Micky's agreement with his current team. In the ensuing argument, in which Micky chastises both factions of his family, Charlene and his trainer leave in disgust. Micky and Dicky spar, brought to an abrupt end when Micky knocks Dicky down. Dicky storms off, presumably to get high again, and Micky's mother chides Micky, only to be sobered when he tells her that she has always favored Dicky. Dicky returns to his crack house, where he says goodbye to his friends and heads to Charlene's apartment. He tells her that Micky needs both of them and they need to work together. After bringing everyone back together, the group goes to London for the title fight. Micky scores another upset victory and the welterweight title. The film jumps a few years ahead, with Dicky, as talkative as ever, crediting his brother as being the creator of his own success.