Any movie with time travel is likely to have a plot hole. That’s because time is an artificial construct that we use to understand cause and effect. But I don’t want to get bogged down in the science of it all. No matter what type of time travel you use, the story will usually feature two types of stories: 1) the closed loop take where whatever came back in time is destined to come back (examples include Terminator and Star Trek IV) or 2) the mutable future where actions can change the timeline (examples include Terminator 2 and Back to the Future). It’s essentially the determinism vs. free will argument, using a sci-fi construct. I’d like to specifically look at Back to the Future and explain how it is clearly the second type of time travel story, thus leaving us with a dangling thread that is never addressed in any of the sequels. Basically, there’s a second Marty McFly out there.
In the final act of Back to the Future, Marty McFly improvises a way to save his friend Doc Brown. Marty has traveled to 1955 after seeing Doc get gunned down by terrorists in his native 1985. When he can’t get Doc to read his note that explains what happens, Marty decides to return to 1985 ten minutes early. Marty returns to 1985 but the car breaks down and he arrives too late to save Doc but in time to see the events of the beginning of the movie play out, watching a version of himself jump in the Delorean and escape the terrorists by going to 1955. We’re meant to think this closes a loop but I’ll explain why it does not.
The original Marty grew up with a loser family, saw original Doc get killed and then travels back in time to when his parents were his age. By interfering with their lives he risks them never getting together and wiping himself out from existing, but ultimately brings them together and returns home. But as we soon learn, Marty has subtly changed the 1985 he returns to. Examples include the Twin Pines Mall now being called the Lone Pines Mall (Marty took out a tree when he traveled back in time) and waking up to successful parents who no longer tolerate Biff’s bullying ways. Also, Doc from 1955 decided to read Marty’s note and it made him wear a bulletproof vest. The world has changed around Marty. We’re later shown and told that this happens in the second movie: Doc and Marty leave Jennifer in an alternate 1985 to fix the past and Doc says everything will change around her (although since they prevent this timeline from happening, it might be more accurate to say everything there is erased from existence). It seems like if you time travel, you retain your memories of your life but everyone else is rewritten.
The point of all that was to explain that the second Marty, the Marty we see at the end of the movie who escapes into the past is not the same Marty. This is a new version. This Marty grew up with successful parents. And when he goes into the 1955 past, there is already the original Marty there. Multiple instances of a person can exist at the same time with time travel. We see two Martys in the second movie, both at the Enchantment Under the Sea dance. Those Martys are only a couple of days apart in age because he travels to 1955 a second time. The point is, the first Marty goes into the past and changes things, ultimately for the better. But when the next Marty goes to the past there is already a Marty there, changing things. And the second Marty would not want to change things by interacting in any way. What are his options? Interfere with his parents and wipe himself out of existence? If he goes to Doc for help, there’s nothing he’d be able to do. Both Martys can’t use the single lightning bolt to power their car and return home. No, that second Marty would grow old in 1955 while the original Marty lives out his life in the improved 1985. The second Marty cannot close the loop because he exists at the same time as the original Marty and therefore cannot repeat the same actions.
Marty screwed himself.