In 1967, one of Star Trek's most iconic episodes "Mirror, Mirror" aired during season two of the original series. A transporter malfunction brought Captain Kirk, Dr. McCoy, Lieutenant Uhura, and Chief Engineer Scotty to a different universe where viewers saw what looked like the familiar starship Enterprise and her crew until some key differences were revealed. Beloved characters had harsh, often evil counterparts distinguished not just by their opposite personalities but changes in appearance that included Spock's alternate famously sporting a goatee.
The episode was the first time fans were introduced to this Mirror Universe where the Federation was nonexistent and in its place was an Empire. The original series would not return there once the characters safely made their way back to their Enterprise, but it would be far from the last time it would be explored in the franchise.
The Mirror Universe reappeared on TV in 1994 in the Deep Space Nine (DS9) season two episode "Crossover." In "Crossover," Major Kira and Dr. Bashir arrive there after encountering a problem traveling through the wormhole. Deep Space Nine writer and producer Robert Hewitt Wolfe recalls the episode coming about during a conversation between himself, co-creator Michael Piller, writer Peter Allan Fields, and executive producer Ira Steven Behr. They were discussing fun ways to incorporate elements from the original series in the show when someone mentioned the idea.
"Then we quickly just went into what we would do, how'd it work, what it would really be about, and talked about the whole idea of what would be the effect of Kirk's visit on that universe," Wolfe says. "One of the things we talked about very early on in Deep Space Nine is that the previous Star Treks had come to places, done stuff, and then left. We were interested much more in thinking about long-term consequences. When you fly away from the planet and don't go back, it's a simplistic illusion that it just works out. We thought it might not."
This meant the team had to figure out what the Mirror Universe might be like after Kirk tries to convince Spock to turn against the ways of the Empire at the end of "Mirror, Mirror." They decided that Spock's actions would in fact lead to the Empire's downfall. Kira's mirror counterpart, the Intendant, explains that Spock's reforms left the Empire unable to defend itself against the Klingon Cardassian Alliance. Humans who were once dominant were now oppressed.
Wolfe said once they understood what the universe had become it was easy to figure out where the characters' counterparts were and what their roles would be. The basic idea was that everyone would be somewhat the opposite of who they were or sometimes a more extreme version of who they were, showing a buried part that might have come out.
"Crossover" was the first of five DS9 episodes (including "Through the Looking Glass," "Shattered Mirror," "Resurrection," and "The Emperor's New Cloak") to either take place in the Mirror Universe or include characters from there. In these episodes viewers would continue to see the world evolve with a growing rebellion challenging the Alliance. The series examined the universe more than any other.
Wolfe said since they had to do about 26 episodes a year, they thought it was "important to mix it up with different episodes and different flavors."
"The Mirror Universe gave us a way to do pirate, swashbuckling tales that we couldn't do in our own universe. That was the attraction of going back once a season or once every couple of seasons. That kind of fun where the consequences for the characters in that universe could be pretty heavy but in ours would be lighter," he said.
The prequel series Enterprise would return to the Mirror Universe in a new way. The two-part season four episode "In a Mirror, Darkly" was based completely there with no characters crossing over from one side to the other. Enterprise writer and producer Michael Sussman said the episode resulted from a few different notions coming together since they were looking to revisit classic elements from the original series. Two previous attempts at Mirror Universe episodes, including one that would have seen the return of actor William Shatner as Kirk, fell through. However Sussman had had the idea for a show that would refer to an original series episode ("The Tholian Web") where a Federation starship goes missing and pitched a version combining it with the Mirror Universe.
"One of the reasons we did this show within a show and didn't do the usual crossover was we wanted to stay true to the 'Mirror, Mirror' episode which was truly the first crossover anybody knew about [in the universe] and didn't want to do something to undercut that storyline which is such a great episode," he explained.
"In a Mirror, Darkly" saw the alternate Enterprise crew serving the Empire, crushing rebels, and discovering the U.S.S. Defiant from the prime Star Trek universe which they soon used to their advantage. Sussman said creating the characters' alternates wasn't easy. They wanted to avoid anything too over the top and make it realistic instead of just making good characters suddenly evil. They wanted to make them grounded with understandable, relatable flaws.
This was the last time the universe was portrayed on TV, but it has continued to reappear elsewhere. In Star Trek video games there are multiple references to the universe, including those in the first-person shooter Voyager Elite Force and the massively multiplayer online game Star Trek Online. The battle simulator Shattered Universe is even set in the Mirror Universe. References can also be found in Decipher's tabletop roleplaying game and the miniatures game Attack Wing.
Multiple books have further fleshed out the universe, from a Kirk-centric trilogy written by Shatner with Judith and Garfield Reeves-Stevens to Susan Wright's Dark Passions duology that focused more on the women of the universe to the thriller Section 31: Disavowed by David Mack. Comics like DC Comics' omnibus The Mirror Universe Saga and a one shot sequel to "Mirror, Mirror" by Marvel Comics also explored it. A new version of the universe even appeared in IDW Publishing's ongoing series written by Mike Johnson set in the J.J. Abrams reboot universe.
"We wanted the comics to have nods to the original episode, like the iconic goatee, but at the same time we didn't want to just re-hash the events of the original with new faces. Instead, we wanted to embrace aspects of the new timeline, like Kirk's revenge against Nero and the relationship between Spock and Uhura, but see them through the dark lens of the Mirror Universe," Johnson said in an email interview.
Through the decades the desire to return to the Mirror Universe has remained, enabling the franchise to explore areas it otherwise couldn't and adding possibilities beyond its usual framework.
"Like the best Star Trek stories, I think it embraces and explores a key part of what it means to be human: our id. For all of Trek's wonderful and vital optimism, humanity will always possess less noble instincts, and 'Mirror, Mirror' is an example of the show's willingness to confront that," Johnson said. "It's such fertile ground for storytelling, which is why new incarnations of the franchise continue to re-visit the idea."
Sussman sees the Mirror Universe and alternate realities as Star Trek's future, pointing to Abrams' reboot as proof of this. The optimistic, hopeful future depicted by Star Trek may be wonderful and fun but Sussman said there's also something about exploring the darker side, the 'what if it wasn't like that?' that people are interested in seeing and are ready for. With so many episodes already set in the prime universe, Sussman also said it has become an era burdened with a lot of backstory and established elements that can be hard to keep track of unless you go far into the future.
"How do you keep inventing in that universe without stepping on any toes? One way is to make an alternate reality," he says. "Why not go dark if you're going to do that [create an alternate reality] anyway? It's not the only future, but one that should be explored maybe ultimately in a TV series."