Here, you take a movie and speculate on it and come up with a ridiculous or over thought conclusion if something happened differently, or happened to characters that were never seen again, ext.
Back to the Future
Marty McFly never returned home. Through his actions in 1955, he created a tangent universe, an Alternate 1985 if you will. Since he never returned to his proper time line (called universe Alpha), Marty will have been assumed murdered after searches fail to recover him. A weighted coffin is buried, and the grief tears apart the McFly family, resulting in the divorce of George and Lorraine, leaving Biff open to catch Lorraine on the rebound.