The midseason premiere of Star Trek: Discovery - the Jonathan Frakes-directed "Despite Yourself" - confirmed one of the show's longest brewing rumors, revealing that the titular Federation starship has unexpectedly found itself in the Mirror Universe following a malfunction of its experimental spore drive.
The Mirror Universe is an alternate reality that runs parallel to the main Star Trek timeline. Something of a dark mirror of the traditionally optimistic, progressive worldview of Star Trek, the Mirror Universe sees the benevolent Federation replaced by the evil Terran Empire, a violent, ruthless organization obsessed with conquest. The alternate reality has its origins in the iconic original series episode "Mirror, Mirror," where a transporter malfunction sends Kirk, Uhura, Bones, and Scotty to the Mirror Universe, where they have to pretend to be their more brutal counterparts and figure out a way to get home while avoiding suspicion from the despotic crew of the ISS Enterprise, most notably a luxuriously goateed Spock. Somewhat surprisingly, considering its iconic status in the franchise's history, "Mirror, Mirror" was the only episode of TOS to feature the Mirror Universe.
Star Trek: The Next Generation would bypass the Mirror Universe entirely, but it played its most prominent role yet on Star Trek: Deep Space Nine. Dr. Bashir and Major Kira are pulled into the Mirror Universe in the second season episode "Crossover." Set 100 years after the events of "Mirror, Mirror," Bashir and Kira discover that the Mirror Universe's exposure to the more altruistic members of the main universe Enterprise led to internal reform of the Terran Empire, but that the Empire's enemies perceived that reform as weakness; the Terran Empire was conquered by a wide Alliance of alien species, with humans and Vulcans relegated to slaves in the aftermath. DS9 would continue to revisit the Mirror Universe during its run, with five total episodes over its seven seasons. Captain Sisko's crew would generally aid in humanity's rebellion against the Alliance, but with the ever present nagging suspicion that things could easily go wrong for humanity again.
Perhaps the Mirror Universe's greatest accomplishment was pulling off a feat no one thought possible - it made Star Trek: Enterprise fun. In the two-part season four episode "In A Mirror, Darkly," the abandoned USS Defiant enters the Mirror Universe - a clever crossover with the TOS episode "The Tholian Web" - and the treacherous crew of the ISS Enterprise scramble to secure the vessel, which is a century more advanced than their technology. "In A Mirror, Darkly" was not only a treasure trove of fan friendly callbacks - including a faithfully recreated TOS Constitution class bridge nearly identical to the one William Shatner roamed in 1966 - it allowed the generally stolid, dull Enterprise cast to let loose and embrace their inner evil hams.
"Despite Yourself" actually hinges on a plot point from "In A Mirror, Darkly." Burnham and the crew of the Discovery recover a Klingon database from a sea of space wreckage, in which they find information on the Defiant and its trip into a parallel universe. They believe if they can figure out how the Defiant got to the Mirror Universe, they might be able to figure out how to get back home. Discovery hasn't been shy about inserting itself into established Star Trek canon, but this is easily its biggest connective tissue to what's come before. It will be interesting to see what other connections to prior stories Discovery touches on before it finds its way back home - assuming it does.
Star Trek: Discovery's first season continues with "The Wolf Inside" January 14th at 8:30 PM ET on CBS All Access.