Back to the Future - Ending Explained

The central twist of "Back to the Future" is that Marty McFly's presence in 1955 inadvertently prevents his parents, George and Lorraine, from meeting and falling in love as they originally did. Instead, Lorraine becomes infatuated with Marty. This creates a grandfather paradox where Marty's own existence is threatened, visually represented by his siblings and then himself fading from a photograph. The resolution of the film involves Marty orchestrating a plan for George to "rescue" Lorraine from Marty's feigned advances at the school dance. This plan goes awry when Biff Tannen intervenes, but George, in a moment of unexpected courage, knocks Biff out, which causes Lorraine to fall for him. They share their first kiss at the dance, securing Marty's future. The second major spoiler is that Doc Brown, who was seemingly killed by Libyan terrorists at the beginning of the film, is saved because he heeded Marty's warning from a letter he wrote in 1955. When Marty returns to 1985, he finds a vastly improved reality: his father is a successful and confident author, his mother is happy and healthy, his siblings are successful, and Biff is a subservient auto detailer. The film ends with Doc returning from the future and whisking Marty and his girlfriend Jennifer away to 2015, setting the stage for the sequel.

One popular fan theory suggests that George McFly knew that "Calvin Klein" was his time-traveling son. Proponents of this theory point to George's love of science fiction and the various anachronistic references Marty makes as evidence that George would have figured it out over the ensuing 30 years. Another darker theory posits that Doc Brown was suicidal before his successful time travel experiment, interpreting his line about standing on the toilet to hang a clock as a euphemism and his willingness to stand in the path of the DeLorean as a sign of his desperation. There is also a theory that Doc Brown murders the Marty of the altered timeline to prevent a paradox, though this has been dismissed by those involved with the film.