The Mirror Universe is one of Star Trek's most intriguing concepts. The franchise has wisely used the Mirror Universe sparingly since it was introduced in Star Trek: The Original Series so that each appearance is an event. The alternate reality where nearly every beloved Star Trek character is the opposite of who they are in the Prime Universe has spanned TOS, Star Trek: Deep Space Nine, Star Trek: Enterprise, and Star Trek: Discovery. The Mirror Universe is also very different from the Kelvin timeline of J.J. Abrams' Star Trek movies, which diverges from the Prime timeline at a fixed point in the 23rd century.
The history of the Mirror Universe runs parallel to the Prime Universe's because both realities co-exist in the same space but in a different dimensional plane. The variances between people from the Prime and Mirror Universes are primarily cultural, with the Mirror counterparts hewing towards malevolence and bigotry. However, there is also at least one distinct physical difference between the two universes: When the USS Discovery was trapped in the Mirror Universe during Star Trek: Discovery season 1, Michael Burnham (Sonequa Martin-Green) noticed that "the cosmos has lost its brilliance" in the alternate reality. Burnham didn't just mean metaphorically; the Mirror version of Captain Gabriel Lorca (Jason Isaacs) needed eye drops due to his photosensitivity to the light in the Prime Universe. While Star Trek's Prime timeline has its share of difficulties, every visit to the Mirror Universe confirms that it's an absolutely terrible place in comparison.
In the Mirror Universe, instead of the United Federation of Planets, humanity formed the Terran Empire and successfully conquered the Alpha Quadrant in the 22nd and 23rd centuries. Thanks to the USS Discovery crossing over to the Mirror Universe and back in 2257, both the Federation and the Terran Empire knew of each other's existence but suppressed this information to prevent further crossovers. The Mirror Universe's Terran Empire was ruled by an Emperor and its Starfleet subjugated most of the other races including Vulcans, Andorians, Tellarites, Orions, and Denobulans. While Federation starships are designated USS (for United Starship), in the Mirror Universe, Terran vessels carry the designation ISS (Imperial Starship). However, the Terran Empire collapsed by the 24th century and was overrun by their enemies, the Klingons and the Cardassians, which formed an alliance along with other worlds like Bajor.
The most iconic symbol of the Mirror Universe remains the Mirror Spock (Leonard Nimoy) who sported a goatee and represents its obvious differences. Indeed, this alternate version of the Vulcan is a pivotal figure in the Mirror Universe's canon because it was under his watch that the Terran Empire ultimately fell for good. The history of the Star Trek Mirror Universe does unfold confusingly since fans experienced it starting with TOS, and then DS9 picked up the story in the 24th century before Star Trek: Enterprise jumped backward 200 years with a prequel tale. Star Trek: Discovery season 1 then returned to the Mirror Universe a decade before TOS before it jumped to the 32nd century and filled in some details of what became of the Mirror Universe in the last 800 years. For clarity's sake, here is what happened in the Mirror Universe chronologically from before First Contact with the Vulcans to what is known about the alternate reality in the year 3189.
How The Terran Empire Rose To Power
On the Mirror Universe's Earth, history unfolded in a mostly identical fashion as in the Prime Universe, except the human race embraced war and fascism as a means to solve problems. Mirror Earth's history involved nations going to war, with the strong overcoming the weak, until the pivotal date of April 5, 2063. This was the day of First Contact with the Vulcans, who landed in Boseman, Montana after Zephram Cochrane (James Cromwell) became the first human to achieve warp flight.
However, instead of a peaceful meeting as had occurred in the Prime Universe, which fans saw in Star Trek: First Contact, in the Mirror Universe, Cochrane immediately executed the Vulcan visitor with a shotgun, believing this was the first step of an alien invasion of Earth. (Cochrane's shotgun later came to be owned by the Mirror Jonathan Archer.) Cochrane's people then raided the starship and executed the other Vulcans aboard. With Vulcan technology, the human race took to the stars, establishing the Terran Empire within the next century. The Terrans conquered and enslaved many of the other races in the Alpha Quadrant, and the xenophobic human-led empire lasted for almost 300 years.
The Mirror Universe In Star Trek: Enterprise (22nd Century)
In 2155, Commander Jonathan Archer (Scott Bakula) mutinied against his Captain, Maximillian Forrest (Vaughn Armstrong), and took command of the Terran Empire's flagship, the ISS Enterprise. Archer's scheme involved taking the Enterprise into Tholian space to retrieve the USS Defiant (NCC-1764), a Constitution-class starship that time-traveled from the 23rd-century Prime Universe to the 22nd-century Mirror Universe in the Star Trek: The Original Series episode "The Tholian Web" (unrelated to the USS Defiant, NX-74205, featured prominently in DS9).
With the Terran Empire in danger of losing a war against its many enemies, Archer's ambition was to use the far more powerful Defiant from the alternate reality to stage a coup where he would become the new Terran Emperor. Although the Enterprise (and Captain Forrest) was destroyed by the Tholians, Archer succeeded in commandeering the Defiant. However, Archer was betrayed and killed by his mistress, Hoshi Sato (Linda Park). Sato then took command of the Defiant and completed Archer's coup, declaring herself Empress at the end of Star Trek: Enterprise's two-parter "In A Mirror, Darkly."
The Mirror Universe In Star Trek: Discovery Season 1 (23rd Century)
100 years later, Star Trek: Discovery season 1's Captain Gabriel Lorca (Jason Isaacs) revealed himself to be a Terran from the Mirror Universe and he used the starship's spore displacement hub drive to return to the alternate reality. However, he wanted Michael Burnham to help him stage a coup to dethrone the Terran Emperor Philippa Georgiou (Michelle Yeoh). Burnham and the crew of the USS Discovery posed as their Mirror Universe counterparts in order to survive being trapped in the parallel dimension.
By 2257, the Terran Empire was the dominant power in the Alpha Quadrant and, under Georgiou's leadership, the humans conquered the Klingons, although a band of alien races formed a rebel alliance against the Terrans. Georgiou also knew about the Prime Universe and the Federation because she had access to the USS Defiant's data files. But Georgiou's gravest threat was Lorca, who nearly succeeded in his coup. At the end of Star Trek: Discovery's four-part saga in the Mirror Universe, Lorca was killed and Georgiou's reign was ended, but Michael Burnham brought the evil doppelgänger of her late Captain Georgiou back with her to the Prime Universe.
Emperor Georgiou posed as her Prime Universe counterpart and helped end the Klingon War of 2256-2257 before joining Section 31. In Star Trek: Discovery season 2, Georgiou helped the USS Discovery and the USS Enterprise defeat the rogue A.I. called Control that took over Section 31 and tried to wipe out all organic life in the galaxy. Out of loyalty to Burnham, Emperor Georgiou traveled with the Discovery 930 years into the future, permanently leaving the 23rd century behind.
The Mirror Universe In Star Trek: The Original Series (23rd Century)
Star Trek: TOS' "Mirror, Mirror" was the first appearance of the Mirror Universe that started it all. Chronologically set a decade after Emperor Georgiou traveled to the Prime Universe, a transporter accident switched the Prime Universe's Captain James T. Kirk (William Shatner) and three of his officers with their Mirror Universe counterparts. While USS Enterprise's Spock quickly imprisoned the evil duplicates of his crewmates, aboard the ISS Enterprise, Kirk had to navigate the dangerous political waters of the Terran Empire, where assassinating your superior officers and rivals is the key to advancement and power.
As Kirk grasped the profound differences between the two universes, he realized that the goateed Mirror Spock harbored grave doubt about the longevity of a fascist regime like the Terran Empire, which was doomed to totally collapse within 240 years. Before switching back to his proper universe, Kirk challenged the Mirror Spock to do what he could to change the destiny of the Terran Empire.
In the greater context of the overall Mirror Universe's history, Star Trek: Discovery season 2 revealed that the Prime Universe's Spock already knew what the Mirror Universe was thanks to his adopted sister Michael Burnham's experiences and from meeting Emperor Georgiou. As part of his lifelong pact never to reveal the truth about Burnham or the Discovery, Spock's swift and decisive actions when he meets Mirror Kirk become recontextualized since the Vulcan knew precisely where this evil version of his Captain came from.
The Mirror Universe on Star Trek: Deep Space Nine (24th Century)
Star Trek: Deep Space Nine continued the story of the Mirror Universe for five episodes over the course of its seven seasons. In the first Mirror Universe episode, "Crossover," Major Kira Nerys (Nana Visitor) and Dr. Julian Bashir (Alexander Siddig) accidentally traveled to the alternate reality of the 24th century, where they learned that the Terran Empire was conquered by an alliance led by the Klingons and Cardassians. The Mirror Spock did take over the Terran Empire a century prior, but in his bid to lead the Terrans toward peace, they became vulnerable and were overrun by their alien enemies. Terrans are now slaves in the 24th century fighting a rebellion against the Klingons and Cardassians. Ironically, whereas Spock's legacy was the reunification of the Vulcans and Romulans in the Prime Universe, the Mirror Spock's attempt at nobility caused the demise of the mighty Terran Empire.
DS9's Mirror Universe saga was intensive, with characters crossing over into both universes, and the action revolved around the Deep Space Nine space station, which was still known as Terok Nor in the Mirror Universe. In "Through the Looking Glass," Commander Benjamin Sisko (Avery Brooks) had to briefly replace his dead counterpart who was the leader of the Terran resistance in the Mirror Universe. Meanwhile, "Smiley," the Mirror Miles O'Brien, emerged as a central character of DS9's Mirror Universe story; Smiley not only recruited the Prime Sisko, but he also stole the plans for the USS Defiant from Deep Space Nine so that the Terran rebels could have a warship of their own.
Word of the Mirror Universe eventually reached the Ferengi Alliance and Grand Nagus Zek (Wallace Shawn) jumped to the Mirror Universe in order to open up business opportunities. Meanwhile, it was revealed that the Terrans' main enemy was the Klingon and Cardassian Alliance's Regent, who turned out to be Worf (Michael Dorn). The Regent was eventually overthrown by the rebels in DS9's final Mirror Universe installment, "The Emperor's New Cloak." However, DS9's Mirror Universe story remained unfinished and left off with the Terran rebels still fighting to bring down the Klingon and Cardassian Alliance. This was the last and furthest chronological point Trekkers have seen of the Mirror Universe, since Star Trek: Discovery season 3 jumped centuries past it into the 32nd century.
The State Of The Mirror Universe In Star Trek: Discovery Season 3 (32nd Century)
Star Trek: Discovery season 3 brought the crew of the eponymous starship, including Emperor Georgiou, to the year 3189, where they reunited with what was left of the United Federation of Planets. At Starfleet Headquarters, Georgiou was personally debriefed by a mystery man named Kovich (David Cronenberg), who is likely an agent of the 32nd century's Section 31 and was uniquely interested in the time-displaced Terran Emperor. During their meeting, Kovich updated Georgiou, who knew nothing of the events seen in Star Trek Deep Space Nine, on the state of the Mirror Universe. The Starfleet agent revealed that the distance between the Mirror Universe and the Prime Universe had been increasing for centuries, which has prevented crossovers. In fact, no one had jumped from either reality for 500 years.
Kovich also informed Georgiou of the fact that the Terran Empire that she once ruled collapsed centuries ago, although he didn't specify the Mirror Spock's central role in its demise. Starfleet then claimed that they identified a chimeric strain on the subatomic level in the Terran stem cell, which explains the Terran predisposition towards malevolence, although Georgiou dismissed the notion. However, immediately after her debriefing with Kovich, Emperor Georgiou began suffering debilitating traumatic flashbacks of her past in the Mirror Universe. What this ultimately means for the Emperor and whether fans will learn more about the fate of the Mirror Universe remains to be seen in Star Trek: Discovery.