Following the conclusion of Back to the Future Part III, "Doc" Brown settled in 1991 in Hill Valley with his new wife Clara, their sons Jules and Verne, and the family dog, Einstein.
Verne, Jules and Marty head back to Wyoming on March 3, 1850, a date that has been carved on a wagon buckboard as the day that Daniel Clayton and Martha O'Brien married while on the Oregon Trail. As it turns out, Martha is a tough frontier woman, while Daniel is a mild-mannered scholar who collects butterflies. When Martha sees Marty, she falls in love with him, and Jules and Verne have to reunite their mother's parents. To complicate matters, Martha is kidnapped by an outlaw, Wild Bill Tannen.