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.
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.
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.
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 ...