Star Trek is a science fiction television series created by Gene Roddenberry that aired on NBC from September 8, 1966, to June 3, 1969.^[1] Though the original series was titled Star Trek, it has acquired the retronym Star Trek: The Original Series (ST:TOS or TOS) to distinguish it from the spinoffs that followed, and from the Star Trek universe or franchise that they make up. Set in the 23rd century,^[2] the original Star Trek follows the adventures of the starship Enterprise and its crew, led by Captain James T. Kirk (William Shatner), his First Officer Mr. Spock (Leonard Nimoy), and his Chief Medical Officer Leonard McCoy (DeForest Kelley). William Shatner's voice-over introduction during each episode's opening credits stated the starship's purpose:

When Star Trek premiered on NBC in 1966, it was not an immediate hit; ratings were low and advertising revenue was lackluster. Before the end of the first season of Star Trek, there were calls in the network for the cancellation of the series because of its low Nielsen ratings.^[3] Desilu head Lucille Ball at that time "single-handedly kept Star Trek from being dumped from the NBC-TV lineup."^[4]

Towards the end of the second season, the show was also in danger of cancellation. Its fans gained a third season; but NBC subsequently moved the show to the Friday Night Death slot, at 10 P.M.^[5] Star Trek was cancelled at the end of the third season, after 79 episodes were produced. However, this was enough for the show to be stripped in syndication, allowing it to become extremely popular and gather a large cult following during the 1970s. The success of the program was followed by five additional television series and eleven theatrical films. Guinness World Records lists the original Star Trek as having the largest number of spin-offs among all television shows in history.