The trilogy finale sends Marty McFly back to 1885, where Doc Brown is stranded in the Old West. Facing outlaws, steam-train stunts, and a race against time, Marty must rescue Doc and return to 1985 before the DeLorean is destroyed. Mixing sci-fi time travel with western adventure and romantic comedy, this installment blends genre traditions into a nostalgic, lighthearted climax.

Från IMDB:
After visiting 2015, Marty McFly must repeat his visit to 1955 to prevent disastrous changes to 1985...without interfering with his first trip.