On October 26, 1985, Dr. Emmett Brown arrives in the DeLorean time machine and persuades Marty McFly and his girlfriend, Jennifer Parker, to travel to the future with him and help their future children, with Biff Tannen witnessing their departure. They arrive on October 21, 2015, where Doc electronically incapacitates 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 and lookalike Marty Jr. to refuse an offer to participate in a robbery with Biff’s grandson Griff, thus saving both of Marty’s children from prison.
Marty switches places with Marty Jr. and refuses Griff’s offer, but Griff goads Marty into a fight, and a subsequent hoverboard chase ensues. Griff and his gang are arrested, saving Marty’s future children. Before rejoining Doc, Marty purchases an almanac, containing the results of major sporting events from 1950 to 2000. Doc discovers it and warns Marty about attempting to profit from time travel. 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 the elderly Biff, who has overheard their conversation and picks up the discarded almanac.
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 witnesses Marty being goaded by his co-worker, Douglas Needles, into a shady business deal, resulting in Marty’s firing. Jennifer tries to escape the house but faints after she encounters her 2015 self. While Marty and Doc attend to her, Biff steals the time machine and uses it to travel to 1955 and give the almanac to his younger self, then returns to 2015. Marty, Doc, and an unconscious Jennifer return to 1985, unaware of Biff’s actions.
The 1985 they return to has changed dramatically: Biff has used the almanac to become one of the wealthiest and most corrupt men in America; changed Hill Valley into a chaotic dystopia; secretly killed Marty’s father, George, in 1973; forced Marty’s mother, Lorraine, to marry him; and had Doc committed to an insane asylum. Marty and Doc deduce that the 2015 Biff took the time machine to change 1985, and Marty learns from the alternate 1985 Biff that he got the almanac on November 12, 1955. Biff tries to kill Marty, but Marty flees and travels to 1955 with Doc, leaving Jennifer on her own front porch.
Marty secretly follows the 1955 Biff, and watches him receive the almanac from his 2015 self. Marty then follows him to the high school dance, being careful to avoid interrupting the events from his previous visit. After several fruitless attempts, Marty finally gets the almanac, leaving Biff to crash into a manure truck. Marty burns the almanac, reversing Biff’s changes to the timeline, as Doc hovers above in the time machine. Before Marty can join him, the DeLorean is struck by lightning and disappears. A Western Union courier immediately arrives and delivers a 70-year-old letter to Marty; it is from Doc, who explains that lightning strike transported him back to 1885. Marty races back into town to find the 1955 Doc, who had just helped the original Marty return to 1985. Shocked by Marty’s sudden reappearance, Doc faints.