There’s a very good reason that Back to the Future isn’t just known as an ’80s classic but as one of the greatest sci-fi movies ever made. It’s more than just a fun-filled nostalgia trip – it’s a cohesive, tightly-written coming-of-age story about science, causality, and personal responsibility. As the story of Marty McFly plays out over the span of a trilogy of films, there’s actually an awful lot to unpack with regard to his general character arc.

After accidentally traveling 30 years back in time, Marty accidentally meets his parents and threatens his own existence. Faced with the task of setting his adolescent parents up or fading away into nothing, Marty helps them realize their love for one another before returning home. However, his actions have unforeseen consequences: when he returns to his present, Marty learns that his influence in the past has actually enriched the lives of each member of his family by helping his parents become more assertive.

It makes for a sweet and memorable ending, but it also forms the basis for perhaps one of the most complex Back to the Future theories there is. Following the logic that the films themselves present, Marty is responsible for more than just one slight tweak to his circumstances: he’s changed himself, too. Moreover, this personality shifting has serious implications for Marty and everyone around him, meaning that a potentially dark twist lies just beneath the surface of the beloved movie trilogy.

Marty’s Meddling In The Past Changes His Personality

Michael J. Fox as Marty McFly in Back to the Future Part 3

Back to the Future makes it pretty clear that Marty’s brief friendship with his own father actually shaped him into a more confident man. However, from a logical standpoint, this would also change Marty: with a more assertive father, it would follow that Marty’s upbringing would be different. He also specifically asks his parents not to punish him for burning their rug, hinting that young Marty was then not subjected to the same level of punishment after his meddling in the past. With a different childhood, it’s reasonable to assume that Marty’s personality would be different after his return to 1985.

In fact, though it isn’t specifically explained, it’s almost certainly the case. Marty starts the first film terrified of rejection, but by the time he has returned to 1985, he’s much more confident. He has also apparently developed an aversion to being called “chicken”, which is a major plot point in both sequels. The change in Marty’s personality is not just a logical extension of the first film’s ending, but it’s something that there’s actual evidence of, even if it’s not explored as a side effect of his time travel.

Marty’s Ending Hints That He’s Changed Again – Why That’s So Dark

Michael J. Fox as Marty McFly and Elisabeth Shue as Jennifer Parker in Back to the Future Part 3

The ending of Back to the Future Part 3 hints that Marty has changed again. On the surface, this appears to be the result of him having learned his lesson about putting his pride to one side after his brush with death in a gunfight, but it’s just as likely due to another personality shift. There’s enough evidence to back this up, too: his ancestor witnessed his prideful duel and disapproved, so it’s not unreasonable to assume that the general idea of backing away from a fight was later instilled in him by his family a few generations down the line.

However, this personality shift makes Marty’s future much more uncertain. His constant fiddling with the timeline seems to have repeatedly rewritten his behavior and personality to make him better adjusted, but there are still nagging concerns. For starters, Jennifer clearly remembers how reckless Marty was, as she’s impressed that he backed down from a challenge. This means that she remembers the way he used to be, and will be aware that he has undergone at least two major personality changes already. It’s also likely that she herself will have undergone a personality shift, meaning that the couple might no longer be as compatible as they were before they started traveling through time.

Of course, the whole point of Back to the Future‘s ending is to show that the future isn’t written in stone and that Marty is ultimately the master of his own destiny. While that’s certainly a nice sentiment, the changes to Marty’s character are enough to cast doubt over the apparently happy ending, as it proves that there are just too many unknowns. For all we know, Marty seems to have accidentally changed his upbringing again, and he might now have violent tendencies, deep-seated depression, or some other unwanted issue – any of which could be enough to ruin his future in another unforeseeable way.