Set in late 1975, Rocky Balboa is a hard-living but failing prize fighter from an Italian neighborhood of Philadelphia, Pennsylvania. Between fights, he works as an enforcer for loan shark Tony Gazzo. The World Heavyweight Champion, Apollo Creed, announces plans to hold a match in Philadelphia during the upcoming United States Bicentennial. However, he is informed five weeks from the fight date that his scheduled opponent, Mac Lee Greene, is unable to compete due to an injured hand. With all other potential replacements booked up or otherwise unavailable, Creed decides to spice things up by giving a local contender a chance to face him. He finds Balboa in the paper, liking his nickname "The Italian Stallion" and his fighting style, being Southpaw.
Rocky meets with promoter Miles Jergens presuming Creed is seeking local sparring partners. Rocky reluctantly agrees to the match, which will pay him $150,000. After several weeks of training, using whatever he can find, including meat carcasses as punching bags, Rocky accepts an offer of assistance from former boxer Mickey "Mighty Mick" Goldmill, a respected trainer and former bantamweight fighter from the 1920s who always criticized Rocky for wasting his potential.
At the same time, Rocky begins a relationship with Adrian, a clerk at the local pet store. He gradually gains the shy Adrian's trust, culminating in a kiss. Her alcoholic brother Paulie becomes jealous of Rocky's success, but Rocky calms him by agreeing to advertise his meatpacking business at the fight. The night before the match, Rocky becomes depressed after touring the arena. He confesses to Adrian that he does not expect to win, but is content to go the distance against Creed and prove himself to everyone.
On New Year's Day, the climactic boxing match begins, 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. Humiliated, Creed takes Rocky more seriously for the rest of the fight, though his ego never fully fades. The fight goes on for the full 15 rounds, with both fighters sustaining many injuries; Rocky suffers his first broken nose and debilitating trauma around the eye, and Creed sustains brutal blows to his ribs with substantial internal bleeding. As the match progresses, Creed's superior skill is countered by Rocky's apparently unlimited ability to absorb punches, and his dogged refusal to be knocked out. As the final round bell sounds, with both fighters locked in each other's arms, they promise to each other that there will be no rematch.
After the fight, multiple layers of drama are played out: the sportscasters and the audience go wild, Jergens announces over the loudspeaker that the match was "the greatest exhibition of guts and stamina in the history of the ring", and Rocky calls out repeatedly for Adrian, who runs down and comes into the ring as Paulie distracts arena security. As Jergens declares Creed the winner by virtue of a split decision (8:7, 7:8, 9:6), Adrian and Rocky embrace and profess their love to each other, not caring about the result of the fight.
Output: Who won the fight, and by what decision?