Star Trek: Discovery's midseason premiere reintroduced one of the franchise's most popular storylines, the Mirror Universe. After Discovery "accidentally" jumps to an alternate universe, the crew does a little investigating only to realize they're in very unfriendly territory. While the Mirror Universe shares many people and elements in common with the Prime Universe, its culture and values are vastly different -- the antithesis, as Michael Burnham says, of everything the Federation stands for. Instead of going into space for the purposes of peaceful exploration and coexistence with other races, the people of that Mirror Earth conquered everything in the known galaxy, and formed the Terran Empire. Their species values racial purity, which is why the rebellion Burnham and the others learn about is made up of a patchwork of other races subjugated by the Terrans.

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Long story short, the Mirror Universe is essentially the Prime Universe's evil twin, and it's always represented a great opportunity to take characters who are normally well-behaved and have a little more fun with them. "Captain Killy," the dark analog of kindhearted Cadet Tilly, is only the tip of the iceberg. The Mirror Universe has a rich history, starting in the Original Series and going all the way through Star Trek: Enterprise, not to mention appearing in related comics and novels. If you're on the casual side of Star Trek fandom, but find yourself curious about this deliciously campy alternate reality, we put together a guide to the Mirror Universe's appearances throughout the history of the franchise.

"Mirror, Mirror"

MIRROR MIRROR STAR TREK

This 1967 episode of the Original Series marked the inception of the Mirror Universe, not to mention the trope that will live in infamy: Bad guys wear goatees. Much of the episode takes place aboard the I.S.S. Enterprise after Kirk, McCoy, Scotty and Uhura are accidentally beamed there in a transporter malfunction. Similar to the Discovery crew, the Enterprise away team spends most of the episode trying to blend in as their Mirror counterparts, all the while containing their shock at the savagery of this world and its inhabitants. Kirk undergoes an assassination attempt by Mirror Chekov almost as soon as he arrives, Mirror Sulu attempts to sexually assault Uhura on the bridge, and Mirror Spock sports a nefarious beard.

RELATED: Star Trek: Discovery Has Been Dropping Mirror Universe Clues All Along

The away team manages to escape back to their own reality (thankfully relieving their Enterprise of its own very poorly behaved Mirror counterparts), but not before Kirk is inspired to mind-meld with Mirror Spock in the hopes of communicating how much better life is in a universe where everyone isn't the worst version of themselves. It was Kirk's hope that the mind meld would induce the Vulcan to effect some kind of change, and Mirror Spock was amenable, considering both men agreed the Empire would eventually grow to an unsustainable size. Unfortunately, Kirk's plan was a little too successful ...

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Deep Space Nine

Star Trek Mirror Universe: Deep Space Nine

After The Next Generation tragically ignored the Mirror Universe*, Deep Space Nine really leaned into the concept. Instead of making its interdimensional travel dependent on random ion storms that would be difficult to replicate plausibly, the episode "Through the Looking Glass" established the existence of an multidimensional transporter that made it much easier for members of the Prime Universe and their Mirror counterparts to visit each other in their respective realities.

RELATED: Star Trek: Discovery May Have Opened the Door to the Mirror Universe

"Crossover" was the first of five Mirror Universe episodes, and in the century between it and the events of "Mirror, Mirror," the political landscape had changed drastically. After the mind meld with Kirk, Mirror Spock actually became leader of the Terran Empire and instituted a program of disarmament. Unfortunately, that left the Empire vulnerable to its similarly vicious neighbors, the Klingon-Cardassian Alliance. As on the Original Series and on Discovery, the Mirror Universe is meant to be the opposite of the Prime Universe in almost every way. So, it made sense that the two races, normally mortal enemies, would be allies in the Mirror Universe.

The Klingons and the Cardassians (and later the Bajorans) would enslave the Terrans after conquering the weakened Empire. Much of DS9's Mirror storyline revolved around the Terran resistance that had sprung up in response to their new status as the Alpha Quadrant's least-powerful species. Highlights included the return of Sisko's wife Jennifer, alive in the Mirror Universe, and the awesomely dark versions Kira Nerys, Elim Garak and later, Emperor Worf. Out of all the series that have done Mirror Universe arcs, DS9 spent the most time there by far.

However, if Discovery is your first introduction to the Mirror Universe, Star Trek: Enterprise's two-parter is where you should start if you want to learn more.

Enterprise

ENTERPRISE MIRROR STAR TREK

Set in the 22nd century, roughly 100 years before the events of the Original Series, Star Trek: Enterprise had its own Mirror Universe two-part special during Season 4. Unlike "Mirror, Mirror" or the DS9 Mirror episodes, "In a Mirror Darkly Parts 1 & 2," took place entirely in the Mirror Universe, with minimal contact with the Prime Universe. The story follows Captain Archer's I.S.S. Enterprise as it navigates a rebellion against the Terran Empire and also happens upon the Constitution-class U.S.S. Defiant from the 23rd-century Prime Universe.

In deft tie-in with the Original Series episode, "The Tholian Web," the Defiant went missing, never to be found. "In a Mirror Darkly" established that it had wound up in the Mirror Universe, and when the Enterprise encounters it, they poach its technology and use it to defeat the rebellion and take over the Empire. We won't spoil the specifics for you, but where "Mirror, Mirror" ends with the Empire about to start its decline, Enterprise's episodes conclude with it about to start a significant upswing, given its recent acquisition of advanced technology.

The circumstances surrounding the Defiant's arrival in the Mirror Universe is the information Burnham, Lorca and Tyler are searching for when they board the I.S.S. Shenzou disguised as their Mirror counterparts.

*While The Next Generation never visited the Mirror Universe during the series or any of the movies, there's a comic book series titled "Mirror Broken" that debuted in 2017 and a 1993 novel called Dark Mirror. Neither are canon, but if you're a TNG fan, they're lots of fun.


Airing Sundays at 8:30 p.m. ET on CBS All Access, Star Trek: Discovery stars Sonequa Martin-Green, Doug Jones, Jason Isaacs, Anthony Rapp, Shazad Latif, Mary Wiseman, Wilson Cruz and Mary Chieffo.