They came up with a story involving a team of Vulcans investigating the Guardian of Forever. In the past, the founder of modern Vulcan logic, Surak, is killed, causing massive changes to the timeline. The Romulans and Vulcans joined forces to attack the Federation, Worf is no longer a crewmember on the Enterprise, and Tasha Yar remains alive. Sarek and the Vulcans by the Guardian are the only people not affected by the timeline change and, in the end, Sarek returns to the past to take Surak's place, restoring the timeline. After Ganino and Stilwell pitched him the new script, Piller suggested to producer Rick Berman that the story, not the script, be purchased. In a meeting, Piller told Ganino that he wanted to make changes to the story, which included the addition of Tasha Yar. Fearing that what they considered a better story would be lost if the changes were made to "Yesterday's Enterprise", Stillwell talked to Piller and pitched their Guardian of Forever story. While intrigued by elements of the story, Piller felt the use of the Guardian was a "gimmick" and wanted The Next Generation to stand on its own. Instead, Piller suggested that they merge the two stories, with Stillwell and Ganino sharing writing credit. Piller suggested that the Enterprise crew would immediately undergo a transformation due to the presence of the older Enterprise, and that Guinan would be integral to the realization that something was wrong. Ganino and Stillwell were given two weeks to complete their new story. Writing Ganino and Stillwell completed their new combined story in about a week. The writers spent hours each day at Stillwell's apartment working over every detail; they felt pressured to write a story Piller would find acceptable, as they wanted to have the opportunity to write the teleplay. The story treatment was turned in on October 10. Piller immediately decided to purchase the story and distributed the treatment to the writing staff while he discussed changes. Piller felt that Data's romantic feelings for Tasha Yar were over the top, and that an alien probe which served as a central part of the story was a cheat in terms of resolving Picard's dilemma. The writer wanted Ganino and Stillwell to enhance Guinan's role and to find another character arc for Tasha Yar. A revised treatment was submitted on October 29, incorporating Piller's changes. The writers were not involved