Doc and Marty have to travel to the future (i.e. our present) in order to keep Marty's kids from wrecking their lives. Complications ensue. . .
The lesser film of the trilogy, but given how good the other two are, this is not a bad thing. The alternate 1985 is harrowing, an apocalyptic riff on George Bailey's vision of a world without him in It's A Wonderful Life. The film's depiction of 2015 is a bit quaint now that it actually is 2015, but it has to be said the film depicts a more hopeful and pleasant future than the terrorism, Trump and Tumblr-ridden mess we ended up with. Still no damn hoverboards, either! Kudos to all the actors playing multiple roles, but especially Thomas F. Wilson, who steals the film from the leads.