Top 20 Star Trek: The Original Series Episodes

The first introduction of the Roman-like Romulans, who had supposedly never been seen before. Thus, it was shocking when they were discovered to look almost exactly like Vulcans.

The episode commonly thought of as the greatest episode of the original series. McCoy, now insane becauses of something with which he injected himself, starts to change time. Kirk and Spock go back in time to 1930s America, where Kirk falls in love with a local woman. However, he must let her die in order to preserve the timeline and to make sure that the U.S. enters World War II.

The original Mirror Universe story, where Kirk, McCoy, Scotty, and Uhura are sent to a world that is the mirror opposite of their own. However, Spock, although evil, is still logical, and he ends up helping them back home.

A classic Klingon episode that introduces the cutest, cuddliest creatures in Star Trek, the tribbles. Also features Harvey Mudd, a recurring character on the show.

The episode that introduced the great villain of the Eugenics Wars, Khan Noonien Singh. It is Kirk and Khan's first battle of wits, which causes the grudge that leads to the second Star Trek movie.

The episode featuring a mysterious spinning cube in space, controlled by an alien who promises to destroy the Enterprise for trespassing. Features Clint Howard, the brother of Ron Howard, as the small alien.