Before I start ranting, let me say that I consider “Back to the Future” a nearly perfect movie, and the entire series gets an “A” from me. But then, I’m a sucker for time-travel stories.
I don’t even mind the number of times that Doc killed Jennifer.
The crux of a good time-travel piece is generating clear and consistent logic, and the first movie did that well. In that world, anything one does in the past will have ramifications in the future and they show us this again and again, from the Lone Pine tree to Doc’s bullet-proof vest.
In the second film, they elaborate on this by explaining that each change is creating an entirely new world. You remember the scene where Doc draws this out on a chalkboard.
Line A shows the original timeline. Line B shows the new universe created when the inciting event happened in the past. Generating the new timeline means the original universe was “erased from existence.” (I love that line!)
Their logic up to this point is fine. Nevermind the whole “a person can’t erase himself” hullaballoo — if you’re going to suspend disbelief for any time-travel story, you have to be willing to let the story set its own rules.
No, the problem is that Jennifer was such a boring character (even after the addition of the sweet Elizabeth Shue) and since the first movie ended with her hopping a ride with Marty & Doc into the future, the writers of movies 2 and 3 were faced with the prospect of having her hanging out in the DeLorean while the guys did all the fun saving-the-world business … or (Heaven forbid) actually creating a personality for her. They chose option C, dumping her at the first available moment and leaving her out of most of the third film.
They leave Jennifer, of course, in alternate 1985, then decide to journey to the past to right Biff’s wrongs. Marty is hesitant about ditching his girl, but Doc assures him that she’ll be fine, because when they straighten things out, she’ll be dropped in the real 1985 as though nothing had ever happened.
But wait. That is contrary to everything else they’ve ever shown us.
When Biff took the Sports Almanac to 1955, he changed everything, right? Original Doc was replaced by Declared-Insane Doc. Original Marty lost his spot to Fatherless-Dropout Marty. And Original Jennifer presumably became an even more boring Jennifer because she wouldn’t have been hanging around with too-cool Marty anymore.
(Side note: there would have actually been two Jennifers in alt. 1985 – the version who rode in the DeLorean and the alt version who grew up in that universe. But since that leads us to mundanity squared, we can feel free to forget all about it.)
Marty and Doc up in 2015 were not immediately effected because the time-space continuum sometimes has a delayed ripple effect as it regenerates, as shown by the disappearing kids in the photo in the first movie. This inexplicable (yet essential, if Marty was going to restore his existence) phenomenon is, again, consistent throughout the movies.
Except for Jennifer.
The DeLorean Duo gave the world plenty of time to redraw itself between the time they regained the Sports Almanac and when, after a full movie spent hanging out in the Old West, they finally returned to 1985. So would anything from the alt. timeline have survived? Of course not. And Original Jennifer? She’d been dumped in the alt-verse. She, and that entire world, were erased.
The only way for our Jenny to be alive and well in Hill Valley 1985 is if she had never hopped that fateful ride in the DeLorean. But not only did she remember visiting the future, she still had the fax in her pocket.
(Side note: in-home faxes in 2015? Does anyone else suspect that Doc funded his Mr. Fusion by stealing all the patents that led to the invention of e-mail?)
There’s only one logical conclusion. For the greater good, Doc Brown lied to Marty when he explained that Jennifer would be fine if left alone in alt 1985. He was well aware that he had killed her off. Then, stricken with guilt, he and his space-locomotive time-hopped to 2015 once more, snatching Jenny in the last few moments that she continued to exist in our timeline. He then planted her in our 1985, a few days before Marty arrived (to give the ripple effect enough time to do its thing), and gullible Marty believed she had always been there.
As did we. Well played, Doc.