Star Trek is a science fiction television series created by Gene Roddenberry that aired from September 8, 1966 to September 2, 1969. Though the original series was just called "Star Trek", it got the nickname Star Trek: The Original Series ("ST:TOS" or "TOS") to make it different from the spinoffs, and from the "Star Trek" universe or franchise they take up. Set in the 23rd century, the first "Star Trek" follows the adventures of the starship USS Enterprise (NCC-1701) 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: