(written from a Production point of view)
Trent Christopher Ganino is the writer who co-wrote the story for TNG's "Yesterday's Enterprise" together with Eric A. Stillwell. Ganino also appeared in Star Trek VI: The Undiscovered Country as Klingon Judge #2.
The story for "Yesterday's Enterprise" was based on a spec script that Ganino wrote and submitted to the TNG writing staff through Michael Piller's open submission policy. Ganino's script featured the Enterprise-C under the command of Captain Robert Garrett – a character named after Ganino's favorite pizza parlor in his hometown of San Jose, California.
In Ganino's spec script, the Enterprise-C accidentally travels twenty-two years into the future, arriving in the current timeline of USS Enterprise-D. In his version, however, there was no time change or alternate future. Instead, Picard and company are faced with the moral question of whether or not to return the ship to its own time period, knowing that the ship will be lost in battle, and if they do, whether or not they should warn Captain Garrett and his crew of this eventuality, running the risk that Enterprise-C and her crew might use this knowledge of the future to alter history once the ship is returned.
A cover letter that Ganino included with his script submission suggested that he was following the TNG Submission Guidelines by creating another Enterprise crew, but if he really had a say in the matter, he would prefer the story involve the original Enterprise featuring Kirk, Spock and other characters from TOS. Years later, after the release of Star Trek Generations, Rick Berman admitted that if he had known in 1989 that they would be making TNG movies in the 1990s, he would have kept "Yesterday's Enterprise" for the first TNG feature film.
During this time, Ganino and Stillwell ran into each other at a studio screening of Star Trek V: The Final Frontier. After the screening, they spent hours at a local Hollywood diner discussing the film. It was then that they decided to team up to write a story together for TNG. Stillwell told Ganino about a story he had been thinking about which involved Sarek and the Guardian of Forever that would involve a time alteration requiring Sarek to travel back in time to replace Surak, the founder of modern Vulcan philosophy, who had been killed accidentally, thus resulting in the time alteration. In the time altered future, the Federation is at war with a united Vulcan-Romulan Star Empire that has already wiped out the Klingon Empire. Ganino liked the idea, and the two writers agreed to work on developing the story together.
During a 1989 convention appearance in San Jose, California, Stillwell ran into Denise Crosby back stage. Crosby suggested to Stillwell that he should write an episode to bring her back to the show. Crosby eventually made her desire to return to the series known to the powers that be at Paramount.
Soon thereafter, Ganino's spec script "Yesterday's Enterprise" arrived on Michael Piller's desk. Piller liked the idea and he wanted to buy the story and have a professional staff writer re-write it. He sent a memo to Rick Berman hoping to convince him to reconsider his opposition to time travel stories. Around this time, the producers had also learned about Crosby's interest in returning to the series. Piller thought they might be able to incorporate a Crosby guest appearance in "Yesterday's Enterprise". When Stillwell, who was serving as script coordinator at the time, heard about this development, he went to Piller and pitched the Sarek/time-altered universe story he and Ganino had been working on. Piller rejected the TOS elements (Sarek, the Guardian of Forever, etc.) as "gimmicks" he wasn't interested in using, but he liked the notion of the character in the present timeline who needed to sacrifice themselves to save the future by going back in time. Piller told Stillwell and Ganino if they could incorporate that element using Natasha Yar instead of Sarek, he would buy the story.
Ganino and Stillwell immediately set to work writing a new story treatment that would combine the elements of their time altered future with Ganino's original spec script concept. From this collaboration emerged the concept which eventually became "Yesterday's Enterprise". One of the significant revisions was the change of Captain Robert Garrett to Rachel Garrett, setting the stage for Tasha Yar to fill her absence on board Enterprise-C after Garrett is killed.
In developing the story, the most difficult element facing Ganino and Stillwell was trying to figure out how Picard and company, in the battle-worn altered reality, would learn the truth about their situation. How would they know that history had been altered? It was Piller who suggested that Guinan might have some mysterious alien intuition that would solve the problem.
Piller convinced Berman to let him buy the story. After two story treatments written by Ganino and Stillwell, Piller assigned the final story re-write to staff writer Ronald D. Moore. The teleplay was later written by Ira Steven Behr, Hans Beimler, Richard Manning, and Ronald D. Moore. Michael Piller also did a considerable amount of writing on the episode, but was denied a screenwriting credit due to Writers Guild rules.