In a freak accident, Marty is taken to 1955, where he meets his parents as teenagers. His presence there throws off the balance of the future and to ensure his eventual birth, Marty must make sure his parents fall in love as scheduled. ... more about Back to the Future
Marty and Doc fix the McFly family in the year 2015 only to find new wrinkles back home in Hill Valley, 1985, which forces them to go back to 1955 once again to fix the future. ... more about Back to the Future Part II