It's funny that the plot of 'Back to the Future II' should be based on altering Marty McFly's future. Wasn't Doc the one who was so staunchly opposed to knowing too much about their future, preferring instead to let things take a natural course? 'Destiny!' he called it. But that is exactly what the sequel is all about, Doc's proposal to altar the future. And this leads not only to bad news for Doc Brown and Marty, but for the your Density? I mean, Destiny? (flashback humor).
The story focuses on Marty McFly's future. Picking up right where we left off in the first movie, Doc informs Marty that in the year 2015, Marty's son partakes in some unfortunate activities with Griff (Biff's grandson) that lead to his arrest and incarceration. While in the future to fix up that little mishap (again, messing with 'Destiny'), Marty picks up a sports Almanac to take back with him. The Almanac contains all sports scores since something like 1955 (why it is only the size of a magazine, I don't know, considering it covers major college and pro sporting event for a whole lot of years).
The Doc, in disgust at Marty's foolish get-rich-quick desires, throws the magazine out (while still in 2015). Unfortunately, Biff, now an old man, gets hold of both the magazine and the Delorian and travels to his young self in 1955. This sets off a change of events in the past so that when Marty and the Doc, now in the future, are ready to go back to 1985, suddenly find themselves in an unfamiliar hell. With Biff changing the past, he also changed the future, creating a desolate, alternate 1985. One where Biff is the richest man in Hill Valley, though still the sleaziest. And where a lot of other things have changed as well. Now, Marty and the Doc have to go back to 1955 and get the magazine from Biff if they expect to restore the future and erase the alternative 1985.
This is a great sequel to a great movie. You get the 1989 version of the future (I don't know that 2015 will make the kind of progress we see in the movie with cool flying cars and dehydrating pizzas and hoverboards). This is the special effects and visual beauty of the second, whereas in the first one, it was recreating the past. Marty had to once adapt to 1955, now he has to do the same for 2015, even if only for a moment.
But, it also ties in another creative aspect: when Marty and the Doc must return to 1955, they only know the whereabouts of Biff based on where they last saw him in that year--the school dance and all of that which took place in the first movie. Going back to that past means that a Marty "Calvin Klein" McFly is already there, and the events are taking place again just as we saw them in the first movie. And now, the Marty and the Doc from the future are intermingling once again with their past versions of themselves. So, in essence, the filmmakers had to recreate some of the scenes from the old movie, from different angles, and the actors had to play dual roles (which they do often throughout the trilogy) by being added into those scenes. It was a great special effects/visionary project to undertake, and what makes the series so damned creative and really a fun idea. And here, too, the goal is to avoid running into your past self because, yes, it could altar events once again. I wonder how the future changed since Marty and Doc's intervention in 2015?
So, prepare yourself for what may arguably be the best movie out of the trilogy (probably because you get to see the future and past and everything in between; although, I'm still torn between rating the first or the second as my absolute favorite). It is the continuation of a fun first movie, and keeps up the creativity and novelty. I think that was the reason most responsible for its success: the ability to keep offering something new (although some things, are obviously repeated, like the running gag of Marty blacking out and waking up to some version of his mother informing him of what year it is after he tells her what an awful dream he had).
So, sit back and let the Delorean be your guide.