We’re coming down to the wire in the “Best Original Series Episode” competition and it’s a battle between “The City on the Edge of Forever” and “The Trouble with Tribbles.”

It all started a few weeks ago, when fans nominated their favorite TOS episodes. That process gave us eight shows – “Mirror, Mirror,” “Balance of Terror,” “The Trouble with Tribbles,” “The Doomsday Machine,” “The City on the Edge of Forever,” “Amok Time,” “Devil in the Dark” and “Space Seed” – and those were steadily whittled down to the two finalists.

“The Trouble with Tribbles” is one of Star Trek’s most beloved comedy showcases. The late first-season hour finds Captain Kirk (William Shatner) and the Enterprise crew contending with cute, fuzzy, Klingon-hating, rapidly multiplying creatures that wreak havoc aboard the ship. “The City on the Edge of Forever,” meanwhile, ranks as arguably Star Trek’s finest dramatic hour. In it, the Spock (Leonard Nimoy) and Kirk pursue a crazed Dr. McCoy (DeForest Kelley), who has leaped through a time portal and changed the future by saving Edith Keeler (Joan Collins), a social worker and pacifist movement leader who was destined to die.

It’s not too late to cast a vote for the best-ever episode of the original Star Trek. Vote on the Star Trek Facebook page or right here on the StarTrek.com home page. Voting will end on Thursday evening at 11:59 p.m. and the results will be announced on Friday.

And, coming next month, we’ll start a new best-episode competition, this time focusing on Star Trek: The Next Generation.