When Star Trek hit the airwaves on NBC, it was a new kind of show that offered to challenge social norms in space. Sure, the show has that intergalactic edge to it. But creator Gene Roddenberry was a guy who had his ear on social issues of the day.

He used the space backdrop to address them at times. This gets done with the help of a magnificent cast, led by William Shatner, Leonard Nimoy, DeForrest Kelly, Nichelle Nichols, James Doohan, and George Takei.

Let’s take a look at 5 of the show’s top episodes.

The City on the Edge of Forever

Dr. McCoy goes bonkers and he ends up on a strange planet. A device left by a superior civilization turns into a time portal that shows the history of the Earth starting up. Bones jumps into it and goes to the past in this Season 1 episode of Star Trek. Then, the Enterprise vanishes. Now Kirk and Spock, thanks to a tricorder recording, must get to the point where Bones climbed through the portal. The big push is to stop Bones from changing history.

Mirror, Mirror

A transported malfunction, due to an ion storm, causes Kirk, McCoy, Scotty, and Uhura to be placed in a mirror universe. They get aboard a parallel Enterprise. Additionally, this ship is run by some very nasty barbarians. This storm also sent the malicious ones to the real Enterprise. Now, Kirk and his crew must get home before they are discovered by the parallel universe crew members in this Season 2 episode of Star Trek.

Balance of Terror

In another Season 1 episode, we see the Enterprise crew face a battle with the Romulans. This enemy ship decides to make an armed probe of Federation territory. What will Kirk and company end up doing? Mark Lenard guest stars in this episode.

The Trouble with Tribbles

Captain Kirk and crew get to a space station, only to realize they’d been called there by a Federation commissioner. He wants them to protect a shipment of seeds meant for Sherman’s planet. This planet also has its fair share of Klingons. In the midst of all this are tribbles, small creatures that keep multiplying in this Season 2 episode. Character actor William Schallert makes a solid guest star.

Space Seed

This Season 1 episode introduces viewers to Khan, played by Ricardo Montalban. The Enterprise happens to make contact with the SS Botany Bay, an Earth spaceship from the 20th century. On board are genetically engineered humans in suspended animation. Kirk and Spock learn more about Khan, who is the last of Earth’s tyrants. Khan eventually takes over the Enterprise, but not for long.