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.