When Litwak's Arcade closes at night, the various video game characters leave their normal in-game roles and are free to travel to other games. Within the game Fix-It Felix, Jr., the characters celebrate its titular hero but shun the game's villain character, Wreck-It Ralph. At a support group for video game antagonists, Ralph reveals his desire to stop being the bad guy. Back at home, Ralph finds the other characters celebrating their game's 30th anniversary without inviting him. Felix reluctantly invites Ralph to join them, but the others ostracize him, saying he would have to earn a medal, just as Felix does in their game.
At Tapper's, Ralph learns that he can win a medal in the first-person shooter game Hero's Duty. Ralph enters the game and encounters Sergeant Calhoun, its no-nonsense leader. Between game sessions, Ralph climbs the game's central beacon and collects the medal, accidentally hatching a Cy-Bug, one of the game's enemies. The Cy-Bug clings to Ralph as he stumbles into an escape pod that launches him out of the game. Meanwhile, with Ralph missing, a girl reports to Litwak the arcade owner that Fix-It Felix, Jr. is malfunctioning. Since broken games get unplugged, leaving their characters homeless, Felix goes to find Ralph.
Ralph crash-lands in Sugar Rush, a kart-racing game. As he searches for his medal, he meets Vanellope von Schweetz, a glitchy character who takes the medal and uses it to buy entry into a race. King Candy and the other racers refuse to let Vanellope participate, claiming that she is not really part of the game. Ralph helps Vanellope build a kart. At her home in Diet Cola Mountain, an unfinished racing course, he discovers that she is a natural racer.
Back in Hero's Duty, Felix meets Calhoun, who warns that the Cy-Bugs are capable of taking over any game they enter. As the pair searches for Ralph and the Cy-Bug in Sugar Rush, they separate when Felix, enamored with Calhoun, inadvertently reminds her of her fiancé, who had been killed by a Cy-Bug in her backstory. Calhoun finds hundreds of Cy-Bug eggs underground, and Felix becomes imprisoned in King Candy's castle during his search for Ralph.
King Candy hacks the game's code to retrieve Ralph's medal and offers it to him, explaining that letting Vanellope race would be disastrous for both her and the game. Fearing for Vanellope's safety, Ralph wrecks the kart and returns to his own game, but finds that everyone has evacuated, expecting the game to be unplugged in the morning. Ralph then notices Vanellope's image on the Sugar Rush cabinet and realizes she is an intended part of the game, not a glitch.
Ralph returns to Sugar Rush, finds Felix and Vanellope, and asks Felix to fix the wrecked kart. As the race proceeds, the hatched Cy-Bugs attack and Felix, Calhoun, and Ralph battle them. When Vanellope catches up to King Candy, her glitching reveals that he is actually Turbo, a character from an old game, Turbotime, who sabotaged a newer game out of jealousy, causing both to be unplugged. Vanellope escapes from Turbo, who is consumed by a Cy-Bug. The group flees the doomed game, but Vanellope finds she cannot pass through the exit. Calhoun says the game cannot be saved without a beacon to attract and kill the Cy-Bugs.
Ralph heads to Diet Cola Mountain, where he plans on collapsing its Mentos stalactites into the cola at the bottom, causing a blinding eruption that would attract the bugs. Before he can finish, Turbo, merged with the Cy-Bug that had consumed him, carries him away. Ralph breaks free and dives toward the mountain, intending to sacrifice himself to start the eruption on impact. Vanellope in turn uses her glitching abilities to save Ralph. The eruption starts and draws the Cy-Bugs to their destruction, including Turbo. Vanellope crosses the finish line, restoring her memory and status as Princess Vanellope, the game's ruler and lead character, while keeping her advantageous glitching ability. Felix and Ralph return to their game in time for Litwak to see that it still works, sparing it from being unplugged. Calhoun and Felix marry, and the characters of Fix-It Felix, Jr. gain a new respect for Ralph.