I've only created one Back to the Future theory before (click here to read it). But here's a theory that I think is VERY plausible and it surrounds the recasting of the role of Jennifer Parker in Back to the Future II and III.
In the first Back to the Future movie, Jennifer was portrayed by Claudia Wells. In the next two, she was portrayed by Elisabeth Shue.
I know that the REAL reason why Jennifer's role was recast was because Wells' mother had fourth stage lymphoma and she wanted to tend to her instead of returning to her role. But this theory says that the reason she looks different is actually in canon.
You may be thinking that you've already heard this theory before, but I am adding to the theory with a detail you probably never considered.
Let's first take a look at the ages in the movies. Marty and Jennifer are both 17 in 1985. We know that Marty's parents, Lorraine and George, are both 47 in 1985 because they were 17 in 1955.
So, I think it's safe to say that Jennifer's parents could also be 47 in 1985, because it has never been stated how old they are. And since Marty, Lorraine, and George all went to Hill Valley High School, it is also possible that Jennifer and her parents went to Hill Valley High School as well.
In the original Back to the Future, Marty travels to the year 1955 and he stumbles across his 17-year old parents.
However, Marty accidentally stops his parents from properly meeting and falling in love. To make things worse, Lorraine becomes infatuated with Marty. According to Doc, Marty needed to fix things and get George and Lorraine together because if they weren't married, Marty could not have been born to go back in time and screw things up in the first place. This would create a paradox.
Luckily, Marty knows that they kissed for the first time at the Enchantment Under the Sea dance a week after he arrived in 1955. A dance perfect for couples. Like the couple that had Jennifer.
Marty is able to get George and Lorraine to get together right before he completely vanishes from existence.
Upon their getting together, Marty performs "Johnny B. Goode", a Chuck Berry classic.
Eventually, Marty's guitar solo is so insane that everyone ends up looking at him, bewildered.
My theory is that just as Jennifer's parents were about to kiss and truly fall in love, Marty suddenly interrupted them with his rock and roll.
They were about to kiss, but Marty stopped them from falling in love. So the parents left the dance and broke up. Either the mom or the dad found someone else, fell in love with them, and had Jennifer. And this explains why she looks different.
And this theory should remain intact because at one point, we see Jennifer's dad.
And let's take another look at that picture of everyone at the Enchantment Under the Sea Dance.
There are a lot of guys in that dance that look like they could have easily grown up to be Jennifer's father.
And not only that, but this will tie in nicely to the theory where Doc gains the trust of and then kills another version of Marty. Doc knew Marty would return in 30 years.
So he gained the alternate Marty's trust, got him together with Jennifer, befriended him, and sent him millions of years into the past to be boiled alive in lava.
And that's the theory!
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