A pair of aging boxing rivals are coaxed out of retirement to fight one final bout -- 30 years after their last match.
Rated PG-13 for sports action violence, sexual content and language
In their prime, Pittsburgh boxers Henry "Razor" Sharp (Sylvester Stallone) and Billy "The Kid" McDonnen (Robert De Niro) become rivals after two fights, one in which Kid beats Razor and one in which Razor beats Kid, the only defeats of their careers. Before they have a rematch, Razor announces his retirement without explanation, infuriating Kid and costing them a big payday.
Years later, Razor is low on money and working in a shipyard when he's visited by promoter Dante Slate, Jr. (Kevin Hart), who wants Razor to provide a motion capture performance for a video game. It was Slate's father whose shady business dealings ended up leaving Razor in dire financial straits. The soft-spoken Razor wants no part of this stunt. Kid, a showoff who runs a bar and a car dealership, is all for it.
Razor begrudgingly accepts $15,000 needed to care for his ailing former trainer Lightning Conlon (Alan Arkin) and pay his overdue bills. He doesn't even own a TV, so when Lightning moves in, the old trainer can't even watch favorite shows of his like Dancing with the Stars.
At the recording studio, Razor is surprised and taunted by Kid, who was also invited by Slate. The two get into a fight and damage the studio before being arrested. Cellphone footage of the fight is uploaded on YouTube and goes viral, giving Slate the idea of organizing a final grudge match between Razor and Kid, which he will promote as "Grudgement Day."
Kid eagerly accepts. Razor is forced to do so as well, not receiving his $15,000 and then learning he has been fired from the shipyard. At the press conference to announce the grudge match, Razor is approached by his ex-girlfriend Sally Rose (Kim Basinger), who cheated on him with Kid during their youth and ended up becoming pregnant. Now widowed, Sally wants to reconnect with Razor, but he is reluctant.
Razor asks old trainer Lightning to get him back in shape. Kid expects gym owner Frankie Brite (LL Cool J) to train him, but Frankie mocks the fight and is of little help. Kid is approached by his estranged biological son, B. J. (Jon Bernthal), against Sally's wishes. The two begin to bond after B. J. gives Kid helpful advice regarding his technique, and is invited to be his trainer.
Lightning convinces Razor to forgive Sally. He also finds out Razor is blind in one eye. A fight could cause permanent damage, so Lightning and Sally both implore Razor to call it off. At first he agrees, enraging Kid, but ultimately Razor explains that he needs to do this, Kid having cost him what he loved most, boxing and Sally.
Kid takes his grandson Trey to celebrate the news that the Grudge Match is sold out. B. J. believes they're going to a movie, but Kid takes the child to a bar, then leaves him on his own while hooking up with a groupie. Trey ends up accidentally starting Kid's car while he's in the backseat having sex. Kid prevents an accident, but he is arrested, and B. J. is infuriated that he endangered the boy. Kid apologizes to B. J. and gives him a scrapbook that he kept of B. J.'s sports career in school, proving he hadn't been uninterested in him throughout the years. B. J. forgives him.
The Grudge Match arrives, televised and packed. Kid gains the upper hand, severely beating Razor by unknowingly exploiting his blind eye. Upon learning of Razor's condition, however, Kid stops focusing on the eye and helps Razor to his feet. Razor turns the fight to his favor, but likewise helps Kid on his feet after nearly knocking him out. The fight ends and the judges decide it on points. Razor is declared the winner by a very close split decision. He celebrates with Sally and Lightning, while a satisfied Kid enjoys the company of B. J. and Trey, who are proud of what he has accomplished.
In a post-credits scene, Razor has a new TV, on which he and Lightning view a performance by Kid on Dancing with the Stars and quickly deduce that he's having sex with his partner. In another, Slate tries to set up another grudge match between Mike Tyson and Evander Holyfield. Holyfield refuses several increasingly higher offers to fight Tyson before showing interest when Slate offers Holyfield a role in a fourth Hangover film. Tyson, frustrated by this, approaches Slate angrily before the scene ends.