Back to the Future Part III is a 1990 film and the third and final installment of the Back to the Future trilogy. The film is a science fiction western, using the time travel premise of the series to take Marty McFly (Michael J. Fox) and Dr. Emmett Brown (Christopher Lloyd) back to the Old West of 1885.
The story continues in 1955, from the final scene of Back to the Future Part II. Doc, shocked to see Marty back from the future, promptly collapses, leaving Marty to take him back to the Brown mansion to recover. Once Doc revives, Marty and Doc use the information in Doc’s 1885 letter to retrieve and repair the damaged DeLorean. However, as they retrieve the car, Marty spies a tombstone with Doc’s name, dated a few days after the letter; after learning Doc was killed by Biff Tannen’s ancestor, Buford “Mad Dog” Tannen, Marty decides to go back to 1885 and save Doc.
Marty arrives in 1885 in the middle of a United States Cavalry pursuit of Indians; in his attempts to evade the pursuit, the car’s fuel line is torn, and Marty must abandon the car in a cave. While walking to Hill Valley, Marty meets his great-great-grandfather Seamus; to disguise his identity, Marty adopts the name “Clint Eastwood”. In town, Marty runs afoul of Buford and his gang, who try to hang Marty, but Doc saves him. Doc agrees to leave 1885, but discovers that with the fuel tank in the DeLorean empty and the flying circuits destroyed, there is no way to power the car. Doc devises a scheme to use a locomotive to push the DeLorean up to speed. As Doc and Marty explore the rail spur they intend to use, they spot an out-of-control horse-drawn wagon; Doc saves the passenger, who turns out to be Clara Clayton. The two fall in love, finding many common interests, especially the works of Jules Verne. Buford tries to kill Doc at a town festival over a grudge, but Marty intervenes; Buford then goads Marty into a showdown in two days’ time. Consulting the picture of Doc’s tombstone, they note that Doc’s name has disappeared, but the tombstone – and the date upon it – has not changed.
The night before their departure, Doc tells Clara he is from the future; a disbelieving Clara spurns Doc. Heartbroken, Doc returns to the town saloon to get drunk. Marty rides to the saloon and convinces Doc to leave with him; unfortunately, Doc drinks a single shot of whiskey to celebrate his departure and passes out. Buford arrives and tries to call Marty out, hurling insults at him; Marty realizes that his reputation is unimportant and refuses to fight Buford. Doc revives and tries to flee with Marty, but Buford’s gang spots them and captures Doc, forcing Marty to fight to save Doc’s life. Marty first uses a firebox door from a stove as a bullet-proof shield, and then hits Buford in the face with it. During the ensuing fistfight, Buford destroys the tombstone (which vanishes from Marty’s picture), is knocked unconscious, and is then arrested. With Buford no longer a threat, Marty and Doc ride after and steal the locomotive.
Clara, meanwhile, is leaving town on the train when she overhears a salesman discussing a man he’d met in the saloon, utterly despondent about his lost love; realizing the man was Doc, and that he was still in love with her, Clara triggers the emergency brake and runs back to town. She discovers Doc’s model of the time travel experiment and rides out after him. Clara intercepts and boards the speeding locomotive while Doc is climbing his way towards the DeLorean. Doc encourages Clara to climb out to him, intending to bring her with them to 1985. Just then, the locomotive’s boiler explodes; Clara falls off the side of the locomotive and hangs precariously by her dress. Marty passes Doc his hoverboard from 2015; Doc saves Clara, but can’t catch up to the locomotive. The DeLorean disappears through time, while the locomotive roars over the edge of the incomplete bridge and is demolished.
Marty arrives safely in 1985, but the powerless DeLorean is destroyed seconds later when a freight train strikes it head-on. While driving his pickup truck later that day, Marty avoids being goaded into a street race with Needles, thereby avoiding a potential automobile accident. Remembering what Marty’s parents had said in 2015 about that event ruining Marty’s life forever, Jennifer opens a fax message from 2015 and finds the message “YOU’RE FIRED!” erased. Marty takes Jennifer to the time machine wreckage, but as they survey the remains, a steam-powered locomotive equipped with a giant flux capacitor appears, manned by Doc, Clara, and their children, Jules and Verne. Doc gives Marty a gift—a photo of the two of them by the clockworks at the 1885 festival—and explains to Jennifer that their future is for them to create. Doc’s train then converts into a flying train and roars directly into the camera, finishing the trilogy.