Immediately following his triumphant victory over Ivan Drago in Moscow, Rocky (Sylvester Stallone) faces unexpected health complications stemming from the brutal fight. Returning to the United States with his wife Adrian (Talia Shire), brother-in-law Paulie (Burt Young), and trainer Tony "Duke" Evers (Tony Burton), they are welcomed back by Rocky's son, Robert (Sage Stallone).
During a press conference, boxing promoter George Washington Duke (Richard Gant) tries to provoke Rocky into accepting a match against his boxer Union Cane (Michael Williams), who stands as the top-rated challenger. However, Rocky firmly declines the offer.
Upon arriving back home, Rocky and Adrian are shocked to learn that they are financially devastated due to Paulie's misguided decision to grant power of attorney to Rocky's accountant. This poor judgment results in the mismanagement of Rocky’s finances, leading to unfulfilled tax obligations accrued over the last six years and the mortgaging of their mansion for a staggering $400,000. While Rocky's attorney reassures him that additional fights could remedy their financial situation, Adrian urges him to seek medical advice first. It is revealed that Rocky suffers from brain damage due to the traumatic blows he endured during his bout with Drago, which ultimately forces him into an official retirement from boxing.
As a consequence of their financial troubles, the Balboa family must sell their possessions and relocate back to their old working-class neighborhood in Philadelphia. During a visit to Mighty Mick's Gym, a place promised to him by his late trainer Mickey Goldmill (Burgess Meredith), Rocky discovers the gym in a state of disrepair. Inspired by nostalgic memories of his training days with Mickey, he decides to revitalize the gym and takes on the role of a trainer.
Duke continues to press Rocky, enticing him with the proposition of the fight and the promise of financial gain. However, Adrian steps in once more, clearly indicating that Rocky should not proceed with the fight.
In a twist of fate, Rocky meets a young fighter from Oklahoma named Tommy Gunn (Tommy Morrison). Tommy, while raw and undisciplined, possesses a powerful punch, and although Rocky expresses doubts about his experience as a trainer, he eventually agrees to mentor the persistent fighter. They form a bond as Rocky allows Tommy to stay in his basement while they train together.
As Tommy rises through the ranks to become a fearsome contender, Rocky's dedication to training him inadvertently leads to neglecting Robert, who faces bullying at school. While Robert learns to stand up for himself, he becomes increasingly distanced from his family. As Tommy achieves successive victories, drawing closer to the championship, he begins to be regarded as Rocky’s "shadow."
Despite his growing success, Tommy feels pressure from Washington, who lures him with the promise of luxury and a title shot by fueling a rivalry with Rocky. This leads Tommy to abandon Rocky, leaving a bittersweet taste of betrayal. Realizing the strain this has put on his relationship with Robert, Rocky reconciles with his son, vowing to do better as a father.
Ultimately, Tommy claims the heavyweight title with a striking first-round knockout against Cane but is met with disdain from the audience for abandoning Rocky and crediting Washington for his victory. This leads to a confrontation where Washington instigates a challenge, forcing Rocky to decide whether to fight Tommy. A street fight ensues between the former trainer and his protégé, with Rocky initially overwhelmed but ultimately rising to the occasion with the encouragement of memories of Mickey and the support of his family and friends.
With renewed vigor, Rocky defeats Tommy, who is subsequently arrested, while Washington threatens litigation for Rocky's actions. In the months that follow, Rocky and Robert nurture their relationship further, culminating in an emotional visit to the Philadelphia Museum of Art, where Rocky gifts his son Rocky Marciano's iconic cuff-link, a meaningful keepsake passed down to him by Mickey.
