In 1975, heavyweight boxing world champion Apollo Creed plans to hold a title bout in Philadelphia during the upcoming United States Bicentennial. However, he is informed five weeks from the fight date that his scheduled opponent is unable to compete due to an injured hand, and that all other potential replacements are either booked up or unable to get into shape in time. Having already invested heavily into the fight, Creed decides to give a local contender a chance to challenge him.
Creed selects Rocky Balboa, an Italian-American journeyman southpaw boxer who fights primarily in small gyms and works as a collector for a Mafia loan shark, on the basis of his nickname, "The Italian Stallion". Rocky fights in a local Philadelphia fight club, and he won his last fight with Spider Rico. He meets with promoter George Jergens, who tells him Creed has selected Rocky to fight him for the World Heavyweight Championship. Reluctant at first, Rocky eventually agrees to the fight, which will pay him $150,000. Rocky undergoes several weeks of unorthodox training, such as using sides of beef as punching bags.
Rocky is later approached by Mickey Goldmill, a former bantamweight fighter-turned-trainer whose gym Rocky frequents, about further training. Rocky is not willing initially, as Mickey has not shown much interest in helping him before and saw him as a wasted talent, but eventually Rocky accepts the offer.
Rocky begins to build a romantic relationship with Adrian Pennino, a shy woman who is working part-time at the J&M Tropical Fish pet store. Adrian's brother and Rocky's best friend, Paulie, helps Rocky get a date with his sister and offers to work as a cornerman with him for the fight, an offer Rocky turns down. Paulie becomes jealous of Rocky's success, but Rocky placates him by agreeing to advertise the meat packing business where Paulie works for sponsorship as part of the upcoming fight, and both of them reconcile. Rocky trains extensively for the championship fight, while Apollo is unconcerned about the match and puts more effort into promotion than training. The night before the match, Rocky visits the Spectrum and begins to lose confidence. He confesses to Adrian that he does not believe he can win, but strives to go the distance against Creed, which no other fighter has done, to prove himself to everyone.
On New Year's Day, the fight is held with Creed making a dramatic entrance dressed as George Washington and then Uncle Sam. Taking advantage of his overconfidence, Rocky knocks him down in the first round—the first time that Creed has ever been knocked down. Humbled and worried, Creed takes Rocky more seriously for the rest of the fight, though his ego never fully fades. The fight goes on for the full fifteen rounds, with both combatants sustaining various injuries: Rocky, with hits to the head and swollen eyes, requires his right eyelid to be cut to restore his vision, while Apollo, with internal bleeding and a broken rib, struggles to breathe. As the fight concludes, Creed's superior skill is countered by Rocky's apparently unlimited ability to absorb punches and his dogged refusal to go down. As the final bell sounds, with both fighters embracing each other, they promise each other there will be no rematch.
The fight is extremely well received by the sportscasters and the audience. Rocky calls out repeatedly for Adrian, who runs down as Paulie distracts security to help her get into the ring. As Jergens declares Creed the winner by virtue of a split decision, Rocky and Adrian embrace and profess their love for each other, not caring about the outcome of the fight.
