http://tvtropes.org/pmwiki/pmwiki.php/Script/NumberTwo

Fans of Back to the Future know that the basic plotline of Back to the Future Part II was about Marty McFly and Doc Brown finding themselves in an alternate present ruled by Biff Tannen, and going to the year 1955 (where the events of the original movie took place) to Set Right What Once Went Wrong. But did you know that they were originally going to go back not to 1955, but rather 1967?

That is the premise of Number Two, an early script for Back to the Future Part II. From that point on, it begins to bear similarities to Part I — Marty once again interferes in his parents' life, jeopardizing his own birth (Dave and Linda were already born by this time); and the DeLorean's source of electricity needed to travel in time breaks down, forcing Marty and Doc to rely on Doc's past self to get them back to the future.

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You can find the full script here .

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