The film opens with former U.S. Marine Tommy Riordan (Tom Hardy) visiting his father, Paddy (Nick Nolte), a former alcoholic who has become a Christian. Tommy becomes angry about his father’s formerly abusive behavior, and Paddy fails to convince him that he has truly changed. The next day, Tommy enters a gym where he beats a professional fighter named Pete “Mad Dog” Grimes (Erik Apple) unconscious within seconds, in a fight which is filmed via video phone and later uploaded to the Internet where it goes viral. Tommy asks his father to help him train for a winner-takes-all mixed martial arts tournament called Sparta, but only under the condition that they do not try to reconcile their relationship. It is later explained that Tommy enters the tournament in order to give the money he hopes to win to the widow and son of his fallen friend in the Marine Corps.
Meanwhile, Paddy’s older son, Brendan Conlon (Joel Edgerton), a high school physics teacher, is struggling to financially provide for his wife Tess (Jennifer Morrison) and two daughters, and faces the possibility of his home getting foreclosed if he does not pay the bank in three weeks. Despite he and Tess working a total of three jobs between the two of them, Brendan risks returning to his former profession as an Ultimate Fighting Championship fighter battling amateur fighters for money in order to pay the bank in time. Rumors of Brendan fighting in the ring begin spreading amongst his students, and the school, objecting to his participation in such dangerous activities, suspends him without pay. Without knowing Tommy has done so as well, Brendan decides to also enter Sparta with his old friend Frank Campana (Frank Grillo) as his trainer.
Paddy visits Brendan during this time to inform him that Tommy has returned home and in order to reconcile with his son, which he is unable to do. Brendan says he forgives Paddy for all the wrong he has done to him in the past, but that he still does not trust him. And later, after entering the tournament, Brendan discovers that Tommy has also entered. When Brendan confronts Tommy for the first time in several years, it is explained that Brendan eloped with Tess when he was sixteen years old and left the family, leaving Tommy to take care of his mother, who later fell ill and died, by himself. Tommy says that he refuses to forgive Brendan for this and leaves.
Meanwhile, the video of Tommy beating Mad Dog attracts the attention of a Marine in Iraq whose life was saved by Tommy months prior. The information and video of Tommy saving the soldier is shared with the press and Tommy becomes a national hero, gaining a massive fan base with not only viewers, but with the U.S. Marine Corps as well. Paddy tells Tommy that he is proud of him for saving the soldier’s life, but Tommy gets so furious at his father that Paddy has a relapse and becomes severely drunk the next day. Tommy comforts Paddy and the two finally make peace with one another. However, things turn around when Tommy’s records surface from the Marine Corps and reveal that he took absence without leave from the military after he was nearly killed byfriendly fire. It is explained that Tommy took his mother’s maiden name, Riordan, as his own surname in order to evade arrest for his desertion, and that police will take him into custody after the tournament is over.
Over the next few nights, Brendan and Tommy battle some of the toughest names in mixed martial arts, with Tommy usually winning by brutally knocking his opponents unconscious, and Brendan utilizing Brazilian Jiu-Jitsu and defeating all his opponents by submission. At the end, both brothers come face-to-face with each other in final round and unleash all anger they had towards each other. The fight between the two brothers shows Tommy having the upper hand initially, taking down Brendan and punishing him with some ground-and-pound, but in the third round, the tides turn as Brendan catches Tommy with an omoplata armlock. Eventually, Brendan manages to dislocate Tommy’s shoulder and catch him in a rear naked choke. While they both lie struggling on the mat, Brendan tearfully apologizes to Tommy and tells him that he loves him, and after a brief hesitation, Tommy taps out. The film ends with the two brothers exiting the ring, Tommy in tears with Brendan’s arms around him.