"The Trouble with Tribbles" is one of the most fun episodes of the original series. It all begins when Uhura gets a tribble, a hairy little creature that purrs. Much to everyone's surprise, tribbles multiply rapidly. Before too long, they're all over the ship.
In "The City on the Edge of Forever," Kirk and Spock travel back in time to New York City in the '30s. While there, Kirk falls for Edith Keeler, played by Joan Collins. Kirk learns that Edith must die; otherwise, she'll lead a pacifist movement that keeps the U.S. out of WWII long enough for Germany to win.
"Balance of Terror" introduces a classic "Star Trek" foe — the Romulans. In addition to an intense cat-and-mouse game between the Enterprise and the Romulan ship, the episode tackles prejudice and xenophobia. Some Starfleet members question Spock's loyalties when it's revealed that Romulans and Vulcans share a common ancestry.
Without "Space Seed," we wouldn't have "Star Trek II: The Wrath of Khan," arguably the best "Star Trek" movie. The classic episode is Khan Noonien Singh's only appearance on the original series. Khan is a charismatic and dangerous superhuman who proves quite the challenge for Kirk.
A virus that lowers inhibitions spreads among the crew in "The Naked Time." This causes a swashbuckling Sulu to run around the ship, manically challenging everyone with his sword. Thanks to campy, exaggerated moments from nearly every Starfleet member, it's one of the most fun episodes of the original series.