If there's one thing the Star Trek franchise is well known for, it's the parallel universe that has existed since the Original Series broadcast an episode called "Mirror, Mirror" back in 1967. That episode established the existence of a Mirror Universe, which consists of a reality in which Starfleet never rose, but was replaced by the Terran Empire. Over the course of the numerous series set within the Star Trek universe, that Mirror Universe has been explored, which means it has a history as rich and long as that of the Prime Universe.
Almost every series has delved into the Mirror Universe at one time or another, but not every episode from that alternate reality is necessarily the best. Depending on who is in the episode, what happens, and how it relates to what's going on in that series' normal continuity says a lot about what makes those episodes work. We don't have a magic mirror to tell us which episodes are the best, and that's where you come in. Take a moment to look through the list and vote up your absolute favorite. Was it the first one ever made, or a more recent foray into the parallel universe as seen on Star Trek: Discovery? Only your votes can decide, so vote up the best Mirror Universe episodes below!
In a Mirror, Darkly (2)
The second part of the two-part episode follows the destruction of the Enterprise with the crew retreating to the Defiant, which the Mirror Archer sets his sights on to try and take over the Terran Empire, but it is the Mirror Hoshi Sato who ultimately gains the upper hand and replaces him as the Empress of the Terran Empire.
Original Airing: Apr 29 2005
Season: Star Trek: Enterprise Season 4
Episode Number: 19
In the episode that started it all, four of the crewmembers from the USS Enterprise switch places with their counterparts in the mirror universe. In order to survive, they must avoid being discovered for who they are as they make their way back to their own universe while avoiding tipping off the crew of the mirror Enterprise.
Original Airing: Oct 06 1967
Season: Star Trek: The Original Series Season 2
Episode Number: 4
What's Past Is Prologue
Now that Lorca has been exposed as having come from the Mirror Universe all along, he takes up opposition to the Emperor and the two battle for control of both the Charon and the Terran Empire as a whole. During the fight, Lorca is killed while Emperor Georgiou manages to escape to the Discovery with Michael Burnham.
Original Airing: Jan 28 2018
Season: Star Trek: Discovery - Season 1
Episode Number: 13
More than a century after the events of "Mirror, Mirror" from the original Star Trek episode, Doctor Bashir and Major Kira find themselves in the same place. They arrive in a Mirror Universe where the Terran Empire has been replaced by an Alliance of Klingons, Cardassians, and Bajorans who use the human race as slave labor across the galaxy.
Original Airing: May 15 1994
Season: Star Trek: Deep Space Nine - Season 2
Episode Number: 23