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!
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.
When the crew of the ISS Enterprise finds the USS Defiant, they learn that it is a vessel from 100 years in the future. It had last been seen in the franchise in "The Tholian Web" from the original series, but it found a way through both universes via a rip in space.
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.
In "The Tholian Web," The USS Defiant (NCC-1764) vanishes in Tholian space after becoming trapped in Interphase. The episode has nothing to do with the Mirror Universe, but thanks to the Star Trek: Enterprise episode "In a Mirror, Darkly," the episode was retroactively connected to the Mirror Universe. Because of that later connection, the episode technically qualifies for this list.
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.
Jennifer Sisko makes her way from the Mirror Universe over to the standard one to kidnap Jake. The event is difficult and confusing for the boy who sees his long-dead mother come to take him away, though he was aware of her existence from her father. The point was to bring Ben Sisko over to the Mirror Universe to help O'Brien get their version of the Defiant up and running in their battle against the Alliance.
