Visiting the Russian Federation for a series of boxing matches, George "Iceman" Chambers is subsequently framed for possession of cocaine and sent to prison. There, he discovers a series of illegal full-contact Mixed Martial Arts matches dominated by inmate Yuri Boyka. The prison officials arrange these fights and place large side-bets to make a personal profit, often at the expense of the fighters.
Once inside, the warden and legal adviser for Chambers state that if he fights Yuri in the ring, he will likely get an expedited appeal and early release. He initially refuses, but after spending time in demeaning physical labor in the prison's sewer system and experiencing firsthand the brutality of the guards, he reluctantly agrees. He is rescued from both forms of humiliation by a crippled inmate, Crot (Eli Danker). Both fighters train hard for the match, though Chambers still relies on his boxing background while Boyka prepares a series of deadly kicks, throws, and grappling combinations designed to humiliate his opponent in the ring. Prior to the fight however, Boyka's friends force George's cellmate, Steven, who is acting as his cornerman, to spike his water with a light sedative during the fight.
During the fight, Chambers is somewhat taken by surprise at the flurry and variety of Boyka's attacks, but manages to hold his own through the first round by keeping his distance, staying calm, and dealing out painful punching combinations whenever possible. At the end of the first round, Steven slips the drugged water into Chambers' mouth and he takes a few sips. During the second round, he starts to lose his focus and begins taking more and more punishment until Boyka knocks him out with a devastating flying kick. Following the fight, Steven kills himself out of guilt.
After the fight, Chambers (having learnt of the truth from Crot) confronts Boyka, believing that he was responsible for drugging him, and the warden, for going back on the deal. Boyka coldly tells him that he does not need drugs to win, and the warden denies knowledge of the deal. Upon learning of the truth, Boyka is furious, believing himself to be the first practitioner of the "ultimate fighting form,". Chambers asks for a rematch mainly to ensure his release. He makes a bet that he can defeat Boyka to win his freedom, though the officials also offer a substantial cash incentive to make sure that Chambers trains hard.
Chambers seeks help from Crot, who then reveals that he helped train Boyka in his fighting style. Crot is actually a former soldier and martial arts expert who committed murder in order to protect his family. However, he feels this guilt so acutely, as well as a feeling of alienation from his family, that he has resigned himself to permanent confinement. Even so, his desire to see justice for both Chambers and Boyka inspires him to reeducate Chambers in mixed martial arts techniques. Working together, the two practice grapples, holds, and kicks which stretch Chambers' abilities. As the training continues and Chambers improves, the two begin to form a bond and relate their individual philosophy of fighting. Chambers begins to extend his mental discipline from the ring into his private life, while Crot begins catching some of Chambers' ambition, specifically the narrow and slim hope that he might one day be released and reconcile with his family.
Near the end of the training, Crot teaches Chambers a devious leg Joint lock designed to end the fight. He somberly instructs Chambers to use it to its full effectiveness if he gets the opportunity. Since Boyka enjoys brutalizing his opponents and other inmates with a passion that borderlines on sadism, and his mixture of personal pride and high tolerance for pain make it unlikely that he will submit, Crot instructs Chambers not to hesitate and to break Boyka's leg to end the fight if he gets the chance. Chambers agrees, but privately believes that he can fight Boyka to a knock-out.
Once the fight begins it is apparent to all the viewers that the combatants are much more evenly matched and Chambers is much more impressive in his style. The fight is long and intense, with flurries of combinations, grapples, and throws traded between the two. Eventually, Chambers realizes that Boyka will not lose consciousness, will not submit, and will likely knock him out if the fight goes on too long. Chambers alters his strategy and manages to get Boyka in the joint lock and ends the fight by breaking Boyka's leg, proving that he is the undisputed new champion of the prison.
Shortly thereafter, Chambers is released from prison and uses his winnings to buy Crot's freedom as well. In a final scene, he wheels Crot, who is now cleaned up and in possession of several gifts, to a train station to meet with his estranged niece in a happy reunion. Crot thanks Chambers for giving him the remainder of the winnings to start his life again, while Chambers expresses his gratitude for the help and training. Crot then meets with his niece and the two embrace for the first time.