The Paradox script.
Despite the cliffhanger ending, there were originally no real plans for a Back to the Future sequel. However, once the studio became dead set on making one, director Robert Zemeckis and co-creator Bob Gale agreed to come back to make it. Conceived as a single sequel, a script called Paradox contained elements that were eventually split into II and III. Paradox remained the working title for the sequel shoots and parts of that script were used in the novelizations of the two movies.