On October 26, 1985, Dr. Emmett “Doc” Brown arrives in his flying DeLorean time machine and persuades Marty McFly and his girlfriend, Jennifer Parker, to come back to the future with him to help their future children. Biff Tannen witnesses their departure. They arrive on October 21, 2015, where Doc electronically knocks out Jennifer and leaves her asleep in an alley, explaining that she should not have too much knowledge of future events. He has Marty pose as his own son to refuse an offer to participate in a robbery with Biff’s grandson, Griff.
Marty switches places with Marty Jr. and refuses Griff’s offer, but Griff goads Marty into a fight. Griff and his gang are arrested after they crash into the local courthouse, saving Marty’s future children. Before rejoining Doc, Marty purchases Grays Sports Almanac, a book detailing the results of major sporting events from 1950 to 2000. Doc discovers it and lectures Marty about attempting to profit from time travel, but before Doc can adequately dispose of it, they are interrupted by the police, who have found Jennifer incapacitated and are taking her to her 2015 home. They pursue, as does Biff, who has overheard their conversation.
Jennifer wakes up in her 2015 home and hides from the McFly family. She overhears that her future self’s life with Marty is not what she expected, due to his involvement in an automobile accident. She sees him being goaded into a shady business deal by his coworker, Needles, causing him to be dismissed. Attempting to escape the house, Jennifer encounters her 2015 self and they both faint. While Marty and Doc attend to her, Biff steals the time machine and uses it to travel back to 1955 and give the almanac to his younger self to get rich betting, then returns to 2015. Marty, Doc, and an unconscious Jennifer return to 1985, unaware of Biff’s actions.
The 1985 to which they return has changed dramatically: Biff has become wealthy and corrupt, and has changed Hill Valley into a chaotic dystopia. Marty’s father, George, was murdered in 1973, and Biff has forced Marty’s mother, Lorraine, to marry him. Doc has been committed to an insane asylum.
Doc and Marty find evidence that 2015 Biff used the time machine and deduce that he changed the past. Marty confronts 1985 Biff, who says he received the almanac on November 12, 1955, then reveals that it was he who shot George, as he prepares to shoot Marty. Doc arrives and incapacitates Biff, allowing him and Marty to flee to 1955.
Marty secretly follows 1955 Biff and witnesses him receive the almanac from his older self. Marty then follows him to the high school’s Enchantment Under the Sea Dance, being careful to avoid interrupting the events from his previous visit. Eventually, Biff leaves with the almanac with Doc and Marty in pursuit. In a roadway pursuit, Marty takes the almanac from Biff, who crashes his car once again into a manure truck as Doc and Marty fly away in the time machine.
With Doc hovering above in the time machine as a thunderstorm approaches, Marty burns the almanac on the ground and undoes Biff’s damage to history. Before they can go back to the future, however, the time machine is struck by lightning and disappears. Suddenly, a courier from Western Union arrives and hands Marty a 70-year-old letter from Doc stating that he was sent back to 1885 by the lightning strike. Marty races back into town to find the 1955 Doc who, seconds earlier, has just helped the original Marty from his first time-travel incident go back to 1985. Doc is shocked by Marty’s sudden reappearance and faints.