U.S. Marine Tommy Riordan (Tom Hardy) visits his father, Paddy Conlon (Nick Nolte), a recovering alcoholic who has returned to his Catholic faith. Tommy had to run away from Paddy with his dying mother when he was just a boy due to Paddy being an abusive alcoholic, and has never quite forgiven him. Paddy tries to convince him that he has changed, but to no avail. The next day, Tommy enters a gym where, in less than a minute, he knocks out a professional fighter named Pete “Mad Dog” Grimes (Erik Apple), in a fight which is filmed with a cell phone and later uploaded to the internet, where it goes viral. Tommy learns about a winner-takes-all mixed martial arts tournament called Sparta in which the winner will receive $5 million. Tommy asks Paddy to help him train for the tournament, but only under the condition that Paddy does not try to reconcile their relationship.
Meanwhile, Tommy’s older brother, Brendan (Joel Edgerton), a high school physics teacher and former MMA fighter, is struggling to provide for his wife Tess (Jennifer Morrison) and two daughters. He had to mortgage his house to pay for his younger daughter’s open heart surgery, and is now in danger of losing everything. To increase his income, Brendan battles amateur fighters for money. Rumors of Brendan’s secret life spread around the school, and the superintendent suspends him without pay. Later that night, Paddy comes over to meet up with Brendan in his driveway to try to reconcile with him, but to no avail. Before Paddy leaves, he tells Brendan that Tommy is back in town. Left with no other option, Brendan seeks the training of old friend Frank Campana (Frank Grillo) and begins competing in smaller venue fights. After the fighter Frank planned to enter into the Sparta tournament is injured, Brendan convinces Frank to enter him as a replacement.
After arriving at the tournament, Brendan discovers that Tommy has also entered. It is revealed that Tommy is still angry at Brendan for staying behind for Tess when Tommy and their mother left Paddy; Tommy was left to care for their mother when she became terminally ill. Brendan claims that he could not help having been in love with Tess and that he has forgiven his father for the wrong he has done, but Tommy is unconvinced.
Meanwhile, the video of Tommy beating “Mad Dog” attracts the attention of a Marine whose life Tommy saved in Iraq. The Marine tells the press about Tommy’s heroism, and Tommy becomes a national hero. Tommy’s military records surface and reveal that he deserted after his entire unit was killed in a friendly fire bombing. Tommy had been using his mother’s maiden name as his own surname in order to evade arrest for his desertion. He reveals to have pledged to give his winnings to the widow of one of his fallen friends. The military police will take him into custody after the tournament is over.
Over two nights, Brendan and Tommy have contrasting fortunes: Tommy quickly and brutally knocks out opponents, while Brendan is outmatched physically but uses Brazilian Jiu-Jitsu to force grappling submissions. In the semi-final match, Brendan is matched up with undefeated Russian wrestler Koba (Kurt Angle), who dominates him in two rounds. In round three, Brendan swings for the fences, making the bout a back and forth battle. In the closing moments of the final round, Koba takes Brendan’s back from the clinch and Brendan goes for a rolling kneebar. As Koba attempts to escape the position, Brendan quickly reverses and readjusts the kneebar narrowly winning by submission. Tommy meets Mad Dog Grimes in the semifinals, and Tommy once again knocks him unconscious almost immediately.
The night before the final day, Paddy attempts to talk to Tommy about his actions in Iraq. Tommy angrily dismisses his father, who relapses and starts drinking again. Seeing his terrible pain, Tommy calms and comforts him.
In the end the brothers are the last fighters remaining in the tournament. It is revealed to the world that the two men are brothers. Despite Brendan’s desire to reconcile, Tommy shows no interest in doing so. Tommy wins the first two rounds, but Brendan eventually dislocates Tommy’s shoulder with an omoplata arm-lock. As the fourth round starts, Brendan insists Tommy give up as Tommy only has use of one arm. As Tommy continues trying to knock Brendan out, Brendan unloads a barrage of strikes trying to end the fight. Tommy goads Brendan to continue hitting him as the fourth round ends. At the start of Round 5, Brendan refuses to fight, but Tommy persists. Brendan realizes he has to force him to submit, and he traps Tommy in a rear naked choke. As they struggle on the canvas, Brendan apologizes to Tommy and tells him that he loves him. After some hesitation, Tommy submits. The reconciled brothers exit the ring as their father looks on smiling.