The Back to the Future movies established specific rules for time travel, but one Marty McFly line completely contradicted them. Of course, lines always get a little blurred when it comes to time travel in cinema. It is typically tough for a franchise to maintain its continuity, especially as paradoxes and alternate timelines start to get involved. However, the time travel mechanic of Back to the Future depended heavily on the “changeable timeline” model instead of the paradoxical “whatever happened, happened” method.
