Paddy Conlon, a former Marine recovering alcoholic who has reaffirmed his Catholic faith, and his younger son, Thomas "Tommy" Riordan, a dejected U.S. Marine, travel to Pittsburgh, Pennsylvania, to see his father after 14 years. Tommy's mother was dying, and he had to flee from Paddy as a child because he was a violent drinker. Tommy has never forgiven Paddy yet. To no avail, Paddy tries to persuade him that he has changed. Brendan, Tommy's older brother, now has a family of two kids and his wife, Tess Conlon.
The next day, Tommy walks into a gym. Brendan is a former mixed martial arts fighter who has now turned into a high school physics teacher. One of the top middleweight fighters in the world, Pete "Mad Dog" Grimes, is easily defeated by him and has been recorded by a guy at the gym; Brendan is having financial difficulties supporting his family. He is at risk of losing his Philadelphia home, which he had to mortgage to pay for his younger daughter's open heart surgery.
Paddy trained his son, Tommy, since he was 5 years old to win six straight Junior Olympic titles, and now he recalls the video of it. Brendan begins to wage money fights with amateur boxers to supplement his salary. After learning about the Sparta mixed martial arts tournament, which has a winner-take-all prize pool of $5 million, Tommy asks Paddy to assist him in training for the competition—but only if Paddy agrees to refrain from attempting to mend their relationship. Once word of Brendan’s covert existence got out throughout the school, he was placed on indefinite leave without pay.
To compensate for his wages, Brendan and Tess agree to work on nightclub fights. Tommy’s video of the about rapidly became popular on the Internet. Brendan visits his friend Frank Campana and asks to train him again, even after Frank explains the risk factors. Paddy visits Brendan in his driveway and informs him that Tommy has returned to town, while also attempting reconciliation. Tommy promises to win and give Pilar, the corps friend's widow, the full $5 million purse after that. Brendan starts training under his longtime friend Frank Campana and enters smaller-scale fight competitions. At the same time, Paddy trains Tommy for the Sparta tournament. Brendan persuades Frank to enter him as a replacement after the fighter Marco had intended to enter the Sparta tournament is hurt. And Tess is not happy about Brendan entering Sparta.
Brendan and Tommy get to see each other during the Sparta tournament’s media interview. Paddy notices the footage of Tommy assaulting Grimes and tells the media that Tommy saved his life in Iraq by tearing off the back hatch of a tank that was about to submerge. Brothers meet each other. Tommy is upset with Brendan for abandoning him and staying behind for Tess when Tommy and their mother leave Paddy. Tommy was left to care for their mother when she became terminally ill. This is made clear when the brothers enter the tournament individually. Tommy is not persuaded by Brendan's assertions that he was helplessly in love with Tess, has forgiven his father for his wrongdoings, and has turned away.
Throughout the two battles, Brendan and Tommy have quite different outcomes: Tommy brutally and quickly takes out opponents, remaining silent the entire time, while Brendan, while being physically inferior, employs Brazilian Jiu-Jitsu to force grappling submissions. Tess and the school principal extremely enjoy his fight. Tommy is the target of Paddy's attempt to discuss his conduct in Iraq the night before the last day. Tommy left the Army, according to documents he dug up from the Corps after a friendly fire bombardment killed every soldier in his battalion. Tommy had been trying to avoid being arrested for desertion by going by his mother's maiden name as his surname. Tommy lets all his suppressed feelings fly when he harshly dismisses his father. Paddy goes back to drinking and relapses. Tommy restores his bond with his father by soothing and calming him when he sees him in anguish.
Tommy becomes a national hero, yet according to his military records, he deserted when a friendly firebomb decimated his unit. The Marines come together and sing to honor Tommy during his quarterfinals. According to the commentators, one of the fastest knockouts in MMA history occurs when Tommy fights Mad Dog Grimes in the quarterfinals and nearly knocks him out. On the other side, Brendan’s students and principal enjoy his fight all together in a drive-in theater. Brendan faces off against Russian wrestler Koba, who has never lost before, and Koba wins easily in two rounds.
The fight goes back and forth in the third round as Brendan swings for the fences. Brendan attempts a rolling kneebar in the last seconds of the match, but Koba takes him out of the clinch. Brendan swiftly reverses and readjusts the kneebar while Koba tries to break out of the position, winning by submission by a tiny margin. The government gets to know the truth about Tommy through Pilar’s video, where she discloses that he promised to give her, the widow of one of his deceased pals, Manny, his earnings. He will be arrested by the military police, but they won't release him until the competition is finished.
When Tommy and Brendan are the final two boxers in the tournament, their connection becomes public knowledge. Tommy, who is still in anguish, has no desire to make amends, whereas Brendan expresses a wish to. Tommy prevails in the first two fights, but Brendan uses an omoplata arm lock to finally dislocate Tommy's shoulder.
Although his coach advises against it, Brendan wants to report Tommy's dislocated shoulder to the referee and halt the fight. Since Tommy can only use one arm, Brendan demands that Tommy give up as the fourth round progresses. Tommy keeps fighting as Brendan throws a flurry of strikes in an attempt to win. As the fourth round comes to a close, Tommy encourages Brendan to keep striking him. When Round 5 begins, Brendan knows he must force Tommy to give up, so he traps him with a rear-naked choke. Tommy receives an apology from Brendan and a declaration of love as they battle on the canvas. Tommy hears the words and gives in. Carefully avoiding the referees and the physicians who are checking on Tommy are Brendan and Campana. The brothers walk out of the ring together, their father grinning as he disappears into the background.
