The original idea for "Yesterday's
Enterprise" was generated by
Trent Christopher Ganino in
15 April 1989, as a spec script submitted through the open submissions policy introduced by
Michael Piller in that year. This original version featured the appearance of an
Enterprise from the past in the TNG time period, and the resultant dilemma of whether to reveal their fate before sending them back. In this version, however, no alterations were made to the timeline – the central theme was simply the moral dilemma presented. At this point, the captain of the past
Enterprise was
Richard Garrett, whose last name derived from a pizzeria in Ganino's hometown, San Jose.
At the same time, following a conversation with
Denise Crosby at a convention,
Eric Stillwell developed a story that would allow Tasha Yar to return to the series after a two-year absence. In Stillwell's pitch, drawing from classic
Trek episodes "
The City on the Edge of Forever", "
The Savage Curtain" and "
Mirror, Mirror" – among others – a
Vulcan science team would inadvertently cause the death of
Surak, the founder of
Vulcan philosophy, when a trip through the
Guardian of Forever into
Vulcan's ancient past goes wrong. As a result, the Vulcan people would never become the logical race that is known in the
Trek universe. Instead, a Vulcan race more akin to the
Romulan Star Empire would be engaged in war against the other powers of the
galaxy, including the remnants of the Federation. As part of this alteration, Tasha Yar would be present amongst the crew of the
Enterprise. Ultimately, Ambassador
Sarek, who was on board
Enterprise-D to greet the returning science team, would sacrifice himself by returning to the past and taking the place of Surak, thus restoring the correct timeline. Stillwell commented, "
We thought it would be really cool that someone from the future would replace someone in the past, and I always thought it was funny that their names were so similar anyway." Although not used here, a similar theme would be seen in the
DS9 two-part episode, "
Past Tense, Part I" and "
Past Tense, Part II", where
Benjamin Sisko replaced historical figure
Gabriel Bell.
When Stillwell pitched this idea to Michael Piller, Piller suggested combining it with Ganino's "Yesterday's
Enterprise" story – which had also developed a Tasha Yar element by this point, largely on the suggestion of Piller, who had also wanted to find a way of bringing the character back – with Ganino and Stillwell retaining joint story credit. The Vulcans were replaced by the Klingons, and the Sarek/Surak plotline replaced with the idea that Yar would fill an absence on board
Enterprise-C after the death of a female Captain Garrett. The finished storyline treatment was largely as broadcast, with the exception of Guinan's presence in the episode (in the completed pitch, an alien probe provided the crucial information about the timeline alteration). Some other minor plot points were lost between story and teleplay, including a Yar/Data subplot that was dropped for time.
