Fifty years ago, Gene Roddenberry changed the scope of science-fiction television with the creation of Star Trek.

The series pushed social boundaries and helped define sci-fi as a genre that could show all the possibilities of what humanity could be. The show and franchise became a pop-culture phenomenon thanks to the iconic performances by the show's original cast — William Shatner as Captain James T. Kirk, Leonard Nimoy as Spock, DeForest Kelley as Leonard "Bones" McCoy, George Takei as Hikaru Sulu, Walter Koenig as Pavel Chekov, and Nichelle Nichols as Uhura.

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That original series only lasted three seasons but led to several spin-offs and 13 movies over the past five decades, not to mention influenced an entire genre, gave us several popular catch phrases, created geeky conventions and spawned the phenomena known as Klingon weddings. But Trek isn't done. After over a decade off the air, the Star Trek franchise will continue with the brand new series Star Trek: Discovery, which comes to CBS All Access this January.

To celebrate Star Trek and what it's meant to TV and sci-fi fans around the globe, TVGuide.com put together a list of our five favorite episodes from the original series. And you can catch them during BBC America's marathon of the first two seasons, which starts Thursday at 8:30/7:30c.

Leonard Nimoy and William Shatner, <em>Star Trek</em>Leonard Nimoy and William Shatner, Star Trek

1. "The City on the Edge of Forever" (Season 1, Episode 28)

There's an unending amount of time-travel shows to choose from, but Star Trek helped create the mold with "The City on the Edge of Forever." Spock and Kirk have to travel back to depression-era New York to stop McCoy from completely altering the course of history. If they don't stop him, they risk letting the Nazis win WWII. It drives home one of Star Trek's main themes, that the needs of many outweigh the wants of one and is one of the most beloved episodes in the entire series.

2. "Balance of Terror" (Season 1, Episode 14)

Even if you aren't a Trekkie, surely you've heard of the mean old Romulans, one of the series' greatest antagonists. "Balance of Terror" introduced them to the Trek universe in striking fashion, pitting Kirk and the Enterprise against a Romulan ship outfitted with a cloaking device. The tense hour used the emptiness of space to emulate a sort of sci-fi submarine fight, with Kirk and the Romulan commander engaging in a tactical battle. Of course Kirk wins (because he's the best), earning the respect of his foe even to the degree that the Romulan commander says he would call Kirk a friend in another situation. Then Kirk admirably offers to transport the survivors of the crippled Romulan Bird of Prey to safety, and the commander responds by blowing his own ship up. Romulans, amirite?

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3. "Space Seed" (Season 1, Episode 22)

It's happened to all of us, right? You see something on the street that looks like neglected garbage and you go over to investigate — maybe with plans to find its owner or take it home and give it a new life. Well, that's kind of what happened in this seminal first season episode, wherein Captain Kirk and crew find a vessel, Botany Bay, just floating around space, and Kirk altruistically goes to lend them a hand. Unfortunately, he discovers they're genetically enhanced mutants led by Khan Noonien Singh (Ricardo Montalban) who've been exiled from Earth after some shady antics during a war in the 1990s. Uh-oh! Khan, grabbing control of the Enterprise, seduces Lt. Marla McGivers (Madlyn Rhue) and an awesome tussle ensues between Kirk and Khan. Kirk and co. eventually regain control of the ship — with some help from McGivers — and Kirk sends them to the wild Ceti Alpha V planet. Ultimately, the decision would end up proving to be a fatal mistake — one that does us the favor of setting up the great Star Trek II: The Wrath of Kahn but also eventually cost Spock his life.

4. "Mirror, Mirror" (Season 2, Episode 4)

Notable for practically inventing the concept of the parallel "mirror" universe on TV (you're welcome, Fringe), "Mirror Mirror" sends Kirk, Uhura, McCoy and Scotty into an alternate realm where people are really awful and Spock has a totally fetch hipster Van Dyke goatee situation. There, our heroes find themselves in a place governed by fear, intimidation and backstabbing (like, literal assassination to get ahead) and find a way to get out undetected. Their evil mirror selves, of course, are back in the old world, with a level-headed Spock working to untangle the mess. Eventually, a computer advises Kirk on how to get home — only if he can concoct a plan that has Uhuru, Scotty and McCoy work in concert to pull it off.

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5. "The Trouble With Tribbles" (Season 2, Episode 15)

If only merchandising was as popular in 1967 as it was today! This classic is one of the entire series' most popular, even if it's not a favorite of hardcore Trekkies. When Kirk answers a distress call only to find out the job is to protect a stash of grain (Kirk ain't got time for that!), he's understandably upset. But the real problem comes from an adorable little critter called a Tribble, which is gifted to Uhara by a trader. The little furballs reproduce at an astonishing rate, eventually getting into the ship's systems and eating the grain. But their insatiable appetite uncovers a truth that saves the day: The grain was poisoned by a Klingon spy! Don't worry, several Tribbles made it out alive.

(Full disclosure: TVGuide.com is owned by CBS.)