In "Doomsday Machine," the Enterprise finds another Federation starship, the Constellation, badly damaged and with only one surviving crew member: Commodore Matt Decker. He explains that the Constellation stumbled upon a miles-long machine that "eats" other starships (and basically anything in its path) as fuel; his crew eventually perished after he ordered his ship to attack.

The episode touches on interesting questions about leadership, as well as on weapons of mass destruction and nuclear weapons.

Memorable quote:

Matt Decker: You're bluffing.
Spock: Vulcans never bluff.

Every great show has a great monster battle. And "Arena" is Star Trek's.

The fight scene, featuring a guy wearing what looks like a paper-maché lizard costume, is almost hilariously slow and silly. But, even 50 years later, the episode manages to capture that delightful je ne sais quoi that makes "Star Trek" "Star Trek."

Memorable quote:

Gorn: *Gurgling scream*

In this episode, the crew of the Enterprise encounters a planet fighting a brutal centuries-long war with its neighbor. The catch is that the war is fought entirely through computer simulations, with "casualties" expected to report to suicide chambers when the simulations say they've died.

Memorable quote:

McCoy: In plain non-Vulcan English, we're lucky.
Spock: I believe I said that, Doctor.

Spock begins to suffer from the effects of "pon farr," a powerful Vulcan mating urge. As part of the process of quelling his borderline madness, Spock fights and apparently kills Kirk in an iconic scene.

Bonus: This is the first episode featuring the "Live long and prosper" slogan that has since become a quasi-tagline for "Star Trek."

Memorable quote:

T'Pau: Live long and prosper, Spock.
Spock: I shall do neither: I've killed my captain and my friend.

Spock, McCoy, Scott, and four others make an emergency landing on Taurus II, a planet inhabited with spear-wielding space-orangutans.

The episode is essentially a classic survival and search-and-rescue sci-fi episode, but makes great use of Spock's Vulcan-ness.

Fun fact: This is the first epsiode featuring a shuttlecraft.

Memorable quote:

Spock: Consider the alternatives, Mr. Scott.
Scott: We have no fuel! What alternatives?
Spock: Mr. Scott, there are always alternatives.

"The Corbomite Maneuver" shows the Enterprise encountering a mysterious alien. Kirk then bluffs the alien, claiming that the ship is primed with a powerful self-destruct device that would destroy the aliens should they attack. Eventually, the conflict is resolved peacefully.

Memorable quote:

Sulu: If, eh, anyone's interested... 30 seconds.

This episode features Spock's Vulcan father and human mother amid a backdrop of diplomatic intrigue. Although relations between Spock and his father start out somewhat cold and distant, the two end the episode much closer.

Memorable quote:

Amanda: Logic! Logic! I'm sick to death of logic! Do you want to know how I feel about your logic?
Spock: Emotional, isn't she?
Sarek: She has always been that way.
Spock: Indeed. Why did you marry her?
Sarek: At the time, it seemed the logical thing to do.

Best known as the "episode where Spock has a beard," "Mirror, Mirror" takes the crew into an alternate universe in which the peaceful Federation is instead a brutal and militaristic Terran Empire.

Memorable quote:

McCoy: Jim, I think I liked [Spock] with a beard better. It gave him character. Of course, almost any change would be a distinct improvement.

"Space Seed" gives rise to one of "Star Trek's" greatest villians, the genetically enhanced super-solider Khan.

The episode's real strength is in the setup between the characters: both the Kirk-Khan and the Kirk-Spock dynamics. Plus, it poses questions about the human fascination with and admiration of tyrants and ruthless dictators.

But, more importantly, this episode is essentially a prelude for the best "Star Trek" movie of all time, "The Wrath of Khan."

Memorable quote:

Kirk: He was the best of the tyrants and the most dangerous. They were supermen in a sense. Stronger, braver, certainly more ambitious, more daring.
Spock: Gentlemen, this romanticism about a ruthless dictator is...
Kirk: Mr. Spock, we humans have a streak of barbarism in us. Appalling, but there, neverthless.

Leonard Nimoy put it best here:

"'Devil in the Dark' I thought was a wonderful episode about the fear of the unknown, how we fear and even hate something that we don't know anything about. Learn who your enemy is and maybe that is no longer your enemy."

Memorable quote:

Spock: Curious. What Chief Vanderberg said about the horta is exactly what the mother horta said to me. She found humanoid appearance revolting... but she thought she could get used to it.
McCoy: Oh, she did, did she? Now, tell me, did she happen to make any comment about those ears?
Spock: Not specifically, but I did get the distinct impression she found them the most attractive human characteristic of all.

"The Trouble with Tribbles" is the quintessential fun "Star Trek" episode, featuring a formidable guinea pig-esque foe, the Tribble.

The episode sort of parodies the whole "Star Trek" playbook, and has turned into a cult favorite. Plus, Scotty gets a few great zingers in it.

Bonus: In honor of the 30th anniversary of the show, "Star Trek: Deep Space Nine" inserted its cast into the episode via some 1990s video-editing technology to create "Trials and Tribble-ations."

Memorable quote:

Scott: When are you gonna get off that milk diet, lad?
Chekhov: This is vodka.
Scott: Where I come from, that's soda pop. [Raises glass] Now, this is a drink for a man.
Chekhov: Scotch?
Scott: Aye.
Chekhov: It was invented by a little old lady from Leningrad.

"The City on the Edge of Forever" makes Aeneas' choice between Rome and Dido look like small potatoes.

Our heroes are transported back in time to New York City in the 1930s where our womanizing protagonist, Kirk, falls in love with an idealist and pacifist called Edith Keeler. Soon after, Kirk and Spock find out that Keeler is supposed to die in the near future; otherwise, in an alternate timeline, she mobilizes a pacifist movement in the US, which keeps the country out of WWII long enough for Nazi Germany to develop a nuclear weapon and win the war.

In this brutal but beautifully executed episode, Kirk can either choose to save the noble woman he loves and thereby lead to millions of deaths that would otherwise not have happened, or to let her die so that history will be restored.

Memorable quote:

Kirk: Spock. I believe... I'm in love with Edith Keeler.
Spock: Jim, Edith Keeler must die.

In "Balance of Terror," Captain Kirk and an unnamed Romulan commander (Mark Lenard) engage in a cat-and-mouse game not unlike the submarine battles of World War II. Tensions remain high throughout the episode as each side tries to outmaneavuer the other. Perhaps unsurprisingly, Kirk and the Enterprise crew ultimately prevail.

However, the real significance of the episode is not in Kirk's victory. Rather, it's the idea that there are good people and heroes on both sides of any battle or war or debate — and the hope that human beings will one day recognize that.

Memorable quote:

Romulan Commander: I regret that we meet in this way. You and I are of a kind. In a different reality, I could have called you 'friend.'

The 'Star Trek' movies ranked from worst to best