De Niro and Stallone play Billy 'The Kid' McDonnen and Henry 'Razor' Sharp, two local Pittsburgh fighters whose fierce rivalry put them in the national spotlight. Each had scored a victory against the other during their heyday, but in 1983, on the eve of their decisive third match, Razor suddenly announced his retirement, refusing to explain why but effectively delivering a knock-out punch to both their careers. Thirty years later, boxing promoter Dante Slate Jr., seeing big dollar signs, makes them an offer they can't refuse: to re-enter the ring and settle the score once and for all. But they may not have to wait that long: on their first encounter in decades, their long-festering feud erupts into an unintentionally hilarious melee that instantly goes viral. The sudden social media frenzy transforms their local grudge match into a must-see HBO event. Now, if they can just survive the training, they may actually live to fight again.