Sure, now you think of it as just another one of Marty McFly and Doc Brown’s adventures traveling through time. (By the way, we’re less than two years away from 2015; where are the flying cars, holographic cinemas, and self-drying suits that this movie promised us?) But in 1989, this was an innovative movie in both content and form. In content, the plot forces Marty (Michael J. Fox) and Doc (Christopher Lloyd) to go forward 30 years (where the feud between the McFly and Tannen men replays itself with a new generation), then back to 1955, where Marty essentially watches himself in the first movie while taking further steps to preserve that timeline, the one that ensures that his parents fall in love so that he’ll be born. So the sequel offers a complex and clever running commentary on the original before sending Marty and Doc back to 1885 in Part III for the final, genre-mashup sci-fi-Western retelling of the now-familiar tale.
In terms of form, Part II has that built-in cliffhanger ending because the filmmakers saved money by filming parts II and III at the same time. That was a novel approach back then, but now, it’s common for trilogies. The Matrix filmed its second and third installments back to back, as did Pirates of the Caribbean. Peter Jackson went even further, filming all three Lord of the Rings movies at once, then repeating the feat with all three Hobbit films (the second and third of which are soon to be released). Not only does that approach save some money on production and travel costs, but it also helps ensure continuity, as the actors don’t age several years between installments.