2. The Naked Time
When a mysterious disease begins affecting the mental state of the crew of the Enterprise, Kirk loses control of the ship. As it spirals out of orbit and toward the planet below, McCoy must find an antidote and the ships engines need to be restored before they all run out of time. Leonard Nimoy has said he was particularly proud of his work in this episode. When Spock is afflicted with the virus, the struggle between his Vulcan half and human comes to a head. Spock weeps. And from this episode on, throughout the run of the original series, the films, and the J.J. Abrams reboot, Spocks inner conflictlogic versus emotionis central to his character. While still early in the series run, The Naked Time established the ability of the Enterprise to travel through time, a device that would be used again in the series and more famously in the 1986 film Star Trek IV: The Voyage Home.
Memorable line: "Since the formula worked, we can go back in time, to any planet, any era." Spock