Picking up where Part II left off, Marty McFly is now stranded in 1955. The 1985 Doc is trapped in 1885, but he has written a letter with the explicit instructions that it be delivered to Marty in 1955. The letter explains where the DeLorean is and tells him to immediately return to 1985, where he is to destroy the time machine to prevent further disruption of the space-time continuum.