In their heyday, Pittsburgh's fighters, Henry "Razor" Sharp (Sylvester Stallone) and Billy "The Kid" McDonnen (Robert De Niro), turn into fierce rivals after exchanging victories in the ring—each handing the other his only career defeat. Just before they can reclaim their title in a rematch, Razor unexpectedly announces his retirement, leaving Kid furious and costing them both a lucrative opportunity.
Fast forward a few years, Razor, now struggling financially and working at a shipyard, is approached by promoter Dante Slate Jr. (Kevin Hart), who seeks Razor's involvement in a motion capture project for a video game. It’s Slate's father's unethical dealings that have pushed Razor into his current predicament, making him quite reluctant to participate. Meanwhile, Kid, who owns a bar and a car dealership and revels in the limelight, is eager to jump on board.
Desperate for cash to support his ailing former trainer Lightning Conlon (Alan Arkin), who is at risk of losing his care home due to disorderly conduct, Razor begrudgingly agrees to the gig for $15,000. The fact that he doesn’t even own a television means that Lightning can’t enjoy his favorite programs like Dancing With the Stars when he moves in with Razor.
At the recording studio, Razor is caught off guard by a taunting Kid, who also received an invitation from Slate. Their clash escalates into a wild brawl, causing considerable damage to the studio and resulting in their arrest. The viral video of this altercation sparks the idea of a highly publicized "Grudgement Day" boxing match to settle their differences.
Kid is all in, and despite Razor's unwillingness, he finds himself compelled to participate when he learns he has lost the shipyard job and that the $15,000 is no longer available. During the press event to unveil the match, Razor encounters his ex-girlfriend Sally Rose (Kim Basinger), who had an affair with Kid in their youth. Now a widow, Sally seeks to rekindle their relationship, but Razor is hesitant.
The press conference fails to generate excitement, and Slate urges both fighters to actively promote the upcoming match. Their attempts to sell tickets result in more fights, which only feeds into the hype that Slate is aiming for. Razor finds motivation from Lightning to get back in shape, while Kid relies on self-proclaimed trainer Frankie Brite (L.L. Cool J), who, instead of providing help, merely mocks the event.
In a twist of fate, Kid reconnects with his estranged son B.J. (Jon Bernthal), much to Sally's dismay. Their bond strengthens as B.J. offers invaluable training insights, stepping into the role of Kid’s coach. During this time, Razor discovers he is blind in one eye after a driving mishap, and both Lightning and Sally advise him against going through with the fight, expressing their concerns for his long-term health.
Initially relenting, Razor stirs frustration in Kid. Ultimately, however, Razor realizes he must go through with the fight, motivated not only to avenge his losses but also reclaim his purpose, which was unjustly taken from him—boxing, and Sally's love.
At one promotional event, Razor and Kid’s rivalry manifests as they disparage MMA, only to be confronted by a fighter. Razor's ability to knock him out with a single hit boosts their contention, leading to a sold-out event for their Grudge Match.
As Razor and Sally gradually reconnect, she reveals how his obsession with boxing had made him distant and prompted her betrayal with Kid. Kid spends time with his grandson Trey to celebrate the sold-out match. However, during a night out, he neglects Trey, leaving him alone while he indulges in a brief fling. The careless act leads to an accident but eventually prompts an apology and reconciliation between Kid and B.J.
On the day of the Grudge Match, the arena buzzes with excitement. The fight begins with Kid gaining the advantage, exploiting Razor's blind eye. However, upon discovering Razor's condition, Kid shows sportsmanship, pausing to help Razor up, leading to an honorable exchange of blows. The battle concludes with Razor narrowly winning by a split decision, celebrating the moment with Sally and Lightning, while Kid finds fulfillment with B.J. and Trey.
In a humorous post-credits scene, Razor acquires a new TV to watch footage of Kid's dance performance on Dancing with the Stars, quickly realizing the hilarity of the situation. Another scene teases a potential rematch between boxing legends Mike Tyson and Evander Holyfield, with Holyfield contemplating a comeback thanks to an enticing movie role offer.
