In 1955, teenager Marty McFly discovers that his friend, eccentric inventor Doc Brown, is trapped in the year 1885 in the Old West. Upon this discovery, Marty decides to travel back in time to save him before he meets a premature end at the hands of Biff Tannen's ancestor, Buford "Mad Dog" Tannen. Using the information from Doc's letter and the time-traveling DeLorean left behind for Marty, he and the younger version of Doc from 1955 prepare for a precarious journey to the 19th century. Marty must navigate the challenges of the Wild West, including adapting to the customs and avoiding disrupting the timeline, while aiming to rescue his friend and return to the future.

Upon arriving in 1885, Marty encounters his own ancestors, the McFlys, and realizes he has to tread carefully to not alter his family history. Meanwhile, Doc has settled into life as a blacksmith and is initially resistant to the idea of returning to the future. As the two work together to repair the damaged time machine, complications arise that threaten their mission and lives. They must find a way to power the DeLorean for time travel without the modern convenience of gasoline, while also managing Doc's unexpected romantic entanglement with Clara Clayton, a schoolteacher who is new to the town. This new relationship becomes a defining element of Doc's experience in the past and poses a significant distraction from their goal to return to 1985.