Plot

After visiting 2015, Marty McFly must repeat his visit to 1955 to prevent disastrous changes to 1985...without interfering with his first trip.

Marty McFly has only just gotten back from the past, when he is once again picked up by Dr. Emmett Brown and sent through time to the future. Marty's job in the future is to pose as his own son to prevent him from being thrown in prison. Unfortunately, things get worse when the future changes the present.—FilmFanUk

After a wacky trip to 1955, Marty McFly returns to his own time only to find himself going with his friend Doc Brown to the year 2015 to straighten out a family problem. When they return to 1985, however, they quickly discover Hill Valley is a hellhole. Their only chance to restore their original time is to return to 1955 to set things right.—Blazer346

Lyon Estates, October 26, 1985. Picking up where they left off in Back to the Future (1985), futuristic Doc returns in an enhanced DeLorean and takes Marty to the distant 2015. As a result, to right the wrongs of a grim future and save the present, the time-travellers embark on a frenzied, time-sensitive mission back to where everything started: 1955, the era of drive-ins and ducktails. But a fragile balance governs the space/time continuum; no one tampers with it and gets away scot-free. And this time, jumping back and forth in time has raised the stakes. Can Marty and Doc survive a double dose of temporal displacement?—Nick Riganas

Marty McFly and Doc Brown make an exhilarating visit to the year 2015 seemingly to resolve a few problems with the future McFly family. But when the two return home, they soon discover someone has tampered with time to produce a nightmarish Hill Valley, 1985. Their only hope is to once again get back to 1955 and save the future.—Official Synopsis

On October 26, 1985, Doctor Emmett Brown (Christopher Lloyd) arrives from the future and tells Marty McFly (Michael J Fox) and girlfriend, Jennifer Parker (Elizabeth Shue), he needs help to save their future children from getting into serious trouble. Marty had just home after the adventures of the previous night, and had found a pickup truck in his garage, the kind that he always wanted, indicating that the present was working out as he would have wanted.

Meanwhile, Doc has improved his technology, which now works on a fusion reactor instead of Plutonium and can pretty much run on organic garbage. The car now flies in the air, instead of accelerating to 88 Mph on the road.

As they depart, Biff Tannen (Thomas Wilson) accidentally witnesses their departure. They arrive on October 21, 2015, where Doc electronically hypnotizes Jennifer and leaves her incapacitated in an alley in Hill Valley town to avoid letting her learn about her own future. Doc explains that their son Marty Jr. will be arrested for participating in a robbery with Biff's grandson Griff, leading to a chain of events that destroys the McFly family. Doc has Marty pose as Marty McFly, Jr., Marty's future son, to refuse an offer from Biff's Cybernetically-enhanced grandson, Griff Tannen (Thomas F. Wilson). Marty sees that Marty Jr. is a complete coward and a pushover as Griff bullies him.

Marty successfully switches places with his son and refuses Griff's offer. Griff goads Marty into a fight by calling him "chicken", and a subsequent hover-board chase ensues, which only ends in Griff and his gang crashing into the local courthouse and getting arrested, thus saving Marty's future children. People are still collecting money in 2015 to fix the clock tower. Marty watches on the giant TV screen that the Cubs won against the Miami Giants at 100:1 odd, and one passerby comments that he wished he could go back in time and put some money on the eventual winner.

On his way back to meet Doc, Marty purchases Gray's Sports Almanac, a book detailing the results of major sporting events of the 20th century's second half (1950 - 2000). Doc shows Marty a newspaper from the following day which earlier had a photo of Marty Jr. being jailed for robbery but now has a photo of Griff being jailed for courthouse vandalism. Doc is satisfied that the future has been changed, and Marty's kids are now safe.

Doc discovers Marty's purchase of the almanac and warns him about attempting to profit from time travel, but before Doc can adequately dispose of the almanac, they are forced to follow the police who have found Jennifer incapacitated and are taking her to her future home. Doc is worried that Jennifer coming face to face with her future self could cause a rift in the fabric of space-time and destroy the entire Universe.

Old Biff (who had seen the car leave in 1985 now realizes that it is the same car that has arrived in the future. Biff had also seen the real Marty Jr. coming out of a cafe and realizes that Doc and Marty are visiting the future), overhearing the conversation and recalling the DeLorean from 1985, follows with the discarded book in a taxi.

Jennifer wakes up in her future home and hides while the McFly family has dinner together. She overhears that Marty's life, as well as their life together, is not what they had expected due to a car accident involving Marty. Jennifer witnesses the Marty of 2015 being goaded into a shady business deal by his friend, Needles (Flea), causing their supervisor to fire Marty from his job, as announced by numerous faxes (one copy of which Jennifer keeps). While escaping the house, Jennifer meets her older self and they both faint. As Marty and Doc run to retrieve the younger Jennifer, Biff uses the DeLorean to travel back to 1955, gives his teenage self the sports almanac, then returns to 2015. Marty, Doc, and an unconscious Jennifer return to 1985, unaware of Old Biff's previous actions, and Jennifer is left on the porch at her home.

Marty and Doc soon discover that the 1985 to which they returned has changed dramatically. Biff has become wealthy and corrupt, and changed Hill Valley into a chaotic Dystopia. Marty's father, George (Jeffrey Weissman), was murdered in 1973, and Biff has forced his mother, Lorraine (Lea Thompson), to marry him instead. Doc has been committed to an insane asylum, and Dave, Linda and Marty are away at boarding schools.

Doc finds evidence of the sports almanac and Biff's trip to the past in the DeLorean and tells Marty he needs to learn when the younger Biff received the almanac so they can correct the time line. Marty decides to confront Biff regarding the almanac. Biff explains that he received the book from an old man on November 12, 1955 who told him that he would never lose as long as he bet on every winner in the almanac. He was also told to eliminate anyone in particular who questioned him about the almanac in case of any attempt to change the past. As a result, Biff attempts to kill Marty, during which time he reveals that he killed George and allowed Hill Valley to be taken over by crime to prevent being caught. However, Marty escapes with Doc and, with the new information, returns to 1955.

Marty works undercover to trail the Biff of 1955. Marty is present when the Biff of 2015 arrives to give the Biff of 1955 the almanac, but Marty is unable to retrieve it. Marty is forced, with Doc's help, to try to get the book back during the Enchantment Under the Sea Dance, being careful to avoid undoing the events that he had already corrected in his previous visit. Marty is forced to intervene when Biff's gang goes after the other Marty performing onstage. Eventually, Biff leaves the dance as Doc and Marty follow him silently. After a struggle, Marty takes the almanac from Biff, who crashes his car into a manure truck as Doc and Marty fly away in the DeLorean.

With the storm approaching, Marty burns the almanac and restores the previous (improved) time-line. However, the DeLorean is struck by lightning and disappears. A courier from Western Union arrives minutes later and gives Marty a seventy-year-old letter. It is from Doc, who became trapped in 1885 after the lightning strike made the DeLorean go back to January 1, 1885. Marty races back into town and finds the Doc of 1955, who had just sent the original Marty back to 1985 seconds earlier at the courthouse. Doc is shocked by his friend's sudden re-appearance and faints.