Poor Marty McFly. He's not having a very good day. First his friend Professor Emmett Brown is shot by a bunch of gun-toting crazies, all because he cheated them out of a little plutonium. Then Marty was just barely able to escape in the Prof's new sports car turned time machine.
The problem is that this time machine propelled him back to the year 1955. To make matters worse, Marty managed to interfere with George McFly and Lorraine Baine's first meeting, they are the 50's teenagers that will some day be his parents. Because of Marty, they do not meet as before, which means that they won't fall in love, get married and raise a family. That means Marty will never be born.
Marty has only one hope and that is you! Somehow you have to help him undo the damage he did when he first arrived in the past. You both have to make sure that history repeats itself and his parents fall in love at the big dance. Keep in mind though, that Marty has made lots of enemies among the local bullies. So getting from place to place is going to be a real challenge. And time is definitely not on your side.
--From the NES Back to the Future instruction manual.