In this third instalment of the 'Back to the Future' series, hero Marty McFly (Michael J. Fox) must go back in time to the Wild West of 1885 to rescue his friend Doc Brown (Christopher Lloyd), who he has discovered is due for a fatal showdown with one of bad guy Biff's nasty ancestors. Meanwhile, the good Doc has fallen in love with a newly arrived schoolteacher (Mary Steenburgen).

Teenager Marty McFly (Michael J. Fox) just wants to play electric guitar and date girlfriend Jennifer; he is also determined not to end up a loser in life like his own parents. Father George (Crispin Glover) is a weak-willed failure, bullied at work by his former high school tormentor Biff Tannen (Thomas F. Wilson), while mother Lorraine (Lea Thompson) is an alcoholic. Marty's friend and mentor is the eccentric Doctor Emmet Brown (Christopher Lloyd), who has successfully invented his own time machine in the form of a plutonium-powered Delorean sports car. However, after Marty travels thirty years back in time, he discovers that he doesn't have enough fuel to return to 1985. And, after he accidentally prevents the first ever meeting between his own, teenage parents, Marty's own future birth and present existence is placed in jeopardy!