Back to the Future Part II - Ending Explained

The film’s climax hinges on the burning of the Almanac in 1955, which theoretically resets the timeline. However, the true twist is the Western Union letter delivered in the pouring rain. After the DeLorean is struck by lightning, Marty is stranded in 1955, only for a courier to arrive with a 70-year-old letter from Doc Brown, revealing he was transported to 1885. This reveals that Doc survived and found peace in the past, but the subsequent shock to the 1955 version of Doc (who just sent 'another' Marty home) creates the ultimate temporal loop, requiring Marty to seek help from the very man who thinks he just left.

Critics and fans have often debated the "Marty Replacement" theory: that the Marty we follow at the end of the film is essentially an interloper in a timeline that belongs to a "better" version of himself. Another interpretation suggests the film is a critique of 1980s Reaganomics, with the dystopian 1985 acting as a warning of what happens when unbridled greed and deregulation (represented by Biff) take over an American town. Some viewers also argue that Doc Brown is a 'Time Fugitive', whose attempts to fix the timeline are actually self-serving efforts to avoid his own fated end.