“It means your future hasn’t been written yet! No one’s has! Your future is whatever you make it! So make it a good one! Both of you!”
-Doc Emmett Brown
Back to the Future III

These words, just as this classic trilogy is ending, is the sum of what its creators wanted to say with it. In the first movie, a single act, George McFly standing up to a bully in defense of the woman he loves, alters the lives of McFly and his children, because, at last, he has stood up and fought for something. In the second, Marty McFly makes a terrible, selfish mistake, which is taken advantage of by a petty man, and it ruins the lives of everyone he loves, such that he nearly kills himself putting things right again. In the third, Marty finally comprehends considering the consequences of his actions, and he grows just enough, just in time, to make a different choice from one which would have devastated his life.

Marty’s course is altered by a single choice made in his youth: to be the better man.