even a small miscalculation could destroy the ship, or make them miss their own era. Kirk okays the maneuver, and time on board the Enterprise moves backwards. Christopher is beamed back to his fighter jet at the instant he first encountered the Enterprise, preventing any film evidence of the ship being produced, and erasing his memory of his time on the Enterprise. The same is done with the Air Policeman. Enterprise then successfully returns to the 23rd century. Production The episode was originally conceived as part two to an earlier episode, "The Naked Time"; when the ending to that episode was revised, "Tomorrow is Yesterday" was reworked as a stand-alone story. Associate producer Robert Justman devised the original idea for the story, and it was handed to Dorothy Fontana to create a teleplay. Justman received neither credit nor payment for doing so, whereas Roddenberry's agent charged the studio up to $3,000 for his own stories and rewrites. Reference in further Star Trek stories The method of time travel in this episode was used subsequently to send the Enterprise back to the year 1968 in the second season episode "Assignment: Earth." It is presumed the same technique was used to return the ship to her proper time after the episode ended. The second issue of IDW Publishing's comic book series Assignment: Earth (a continuation of the episode of the same name, drawing on the episode's status as a failed backdoor pilot for a spin-off television series) shows the protagonists, Gary Seven and his assistant Roberta Lincoln, becoming peripherally involved in the events of "Tomorrow is Yesterday", acting on their own to prevent the Enterprises presence from affecting history. Due to peculiarities of time travel, the Enterprise crewmembers have not yet met Seven and Lincoln at this point, but it is Seven's and Lincoln's second encounter with them (though they are careful to avoid direct contact, so as not to alter the proper course of events). The "slingshot maneuver" was employed a third time by the crew in the motion picture Star Trek IV: The Voyage Home, this time to travel back to the year 1986, and to return the Klingon Bird of Prey the command crew had captured at the end of Star Trek III: The Search for Spock to the 23rd century. The Star Trek Star Fleet Technical Manual is described, in two forewords to the manual proper, as having