Paradox was the name of a proposed sequel to Back to the Future that combined the plots of Back to the Future Part II and Part III into one film. However, the producers of Back to the Future decided to not use this draft of the screenplay because it would be too expensive, and rewrote the screenplay as two separate films. Nevertheless, Back to the Future Part II was still shot under the working title Paradox. This was done to try to lessen the amount of publicity.
- Doc uses the sleep inducer on Jennifer before she could ask any questions.
- When Jennifer is in the McFly house, she sees a "videobook" of her and Marty's wedding.
- Future Marty and his kids come over to the older Jennifer after she meets her younger self and find her lying there. (This scene was filmed and deleted.)
- Old Biff meets Young Biff at the high school parking lot during the Enchantment Under the Sea dance.
- When Young Biff confronts Marty, he slips brass knuckles onto his hands.
- Marty takes his jacket off when he is about to confront Biff.
- When Marty's past self opens the door, he instead knocks out Biff. Marty thanks his past self, who says "You're welcome" without looking before running off.
- Marty burns the almanac on the high school football field. This is where the DeLorean time machine disappears and where the Western Union man comes in.
- In the Part III period of the film, Doc was buried in Oak Park Cemetery rather than the Boot Hill Cemetery.
- Marty goes back to April 11, 1888.
- Buford Tannen's nickname is "Black Biff". (Presumably that's where Biff got his name.)
- Tannen demands Marty's name after he nearly hanged him.
- The ravine is called Carson Ravine.
- Heartbroken Doc gets drunk and passes out from drinking an entire bottle of whiskey (apparently he can hold his liquor in this draft).
- Marty first meets his ancestor Seamus in the bar while Tannen is outside waiting for "Clint Eastwood". According to the script, Seamus is the spitting image of "either George or Marty."
- Seamus is not yet married, but a saloon girl who looks like Lorraine is eyeing him with interest.
- Tannen broke his hand after the fight, and is offered a job by Seamus on his farm "sloppin' hogs and washin' wagons". (Had Seamus been played by Crispin Glover, this would reek of history repeating itself in the past, since Biff works for George McFly in 1985.)
- When Marty returns to 1985, the railroad tracks go by the Hill Valley Airport instead of Hilldale.
- Doc had already met Clara prior to sending Marty a letter. There is seemingly no connection between Clara and the ravine.
Throughout the Paradox screenplay, there are many details about items not seen or known about. These items do not conflict with existing canon from Back to the Future Part II & Part III.
- Air traffic control comes on the DeLorean's radio and reprimands Doc for "unauthorized entry into commercial transport airspace".
- Not only is Doc a blacksmith, but also helps out teaching science at the schoolhouse.
- The boy who hands Marty his gunbelt is named D.W. Griffith.