U.S. Marine Tommy Riordan (Tom Hardy) returns home to visit his father, Paddy Conlon (Nick Nolte), a recovering alcoholic who has embraced Christianity. However, Tommy is filled with anger over Paddy's abusive past and is unconvinced of his father's transformation. The very next day, Tommy storms into a gym and displays his formidable strength by knocking out professional fighter Pete "Mad Dog" Grimes in under thirty seconds; this stunning fight is captured on a cell phone and soon goes viral.
Eager to provide for the family of his deceased friend from the Marine Corps, Tommy learns about a life-changing mixed martial arts tournament called Sparta, where the winner claims a staggering prize of $5,000,000. Reluctantly, he approaches Paddy for training, agreeing only if Paddy does not attempt to repair their fractured relationship.
Simultaneously, Paddy's older son, Brendan Conlon (Joel Edgerton), faces his own struggles. As a high school physics teacher and former UFC fighter, he grapples with financial hardships that threaten his family's stability due to hefty medical bills for his younger daughter's heart surgery. To make ends meet, Brendan is compelled to return to the ring, risking his job after being suspended without pay by the school superintendent for participating in the dangerous world of mixed martial arts. He enlists the help of his friend Frank Campana (Frank Grillo) and begins fighting in smaller venues. When a fighter in the Sparta tournament becomes injured, Brendan persuades Frank to enter him as the new contender.
Upon arriving at the tournament, Brendan is confronted with the reality that his brother Tommy has also signed up. Tension simmers beneath the surface as Tommy harbors resentment towards Brendan for abandoning their mother during a difficult time when she battled terminal illness. Although Brendan claims to have forgiven their father, Tommy’s bitterness remains palpable.
As their journeys unfold, the infamous video of Tommy defeating "Mad Dog" garners significant attention; a Marine in Iraq, whose life Tommy saved, shares the footage with the media. This turns Tommy into an unexpected national hero, gaining him support from fans and the U.S. Marine Corps. However, his past catches up with him when military records emerge, exposing his desertion after his entire unit perished in friendly fire, leading to impending arrest post-tournament.
Over two intense evenings of competition, the brothers have contrasting experiences: Tommy quickly overwhelms his opponents, while Brendan, though physically outmatched, relies on his Brazilian Jiu-Jitsu skills to secure victories. Brendan manages to force the Russian champion Koba (Kurt Angle) to tap out in the semi-finals. Tensions escalate the night before the final, resulting in Paddy's return to drinking after failing to connect with Tommy, yet Tommy ultimately shows compassion and comfort towards his father.
In the final showdown, the brothers are the last fighters standing, unleashing years of pent-up emotions and grievances. Initially, Tommy holds the advantage, but an injury shifts the tide. Brendan dislocates Tommy's shoulder, yet Tommy refuses to surrender. In a poignant moment, as they both strain for victory on the mat, Brendan tearfully expresses his love and regret before Tommy ultimately submits. The brothers emerge united from the ring, with Paddy watching on proudly, symbolizing a bittersweet reconciliation born from conflict and understanding.
