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Star Trek's Mirror Universe explained

In the iconic narration of Star Trek: The Original Series (TOS), Captain James T. Kirk vows to go "where no man has gone before." Numerous television series and films later, not only does Kirk make good on his promise, but he's joined in his quest to map the unknown by other heroic captains and crews. Whether by accident or on purpose, Star Trek's heroes have found themselves lost in distant corners of the galaxy, tossed on the eddies of time, caught in the crossfire of wars waged on other planes of existence, and once — no, really — in Sherwood Forest. 

One of the stranger locations that Star Trek keeps bringing its heroes back to is known as the Mirror Universe. The brutal locale makes its first appearance in "Mirror, Mirror" — a season 2 episode of TOS airing in 1967. It would take 27 years for Trek heroes to make their way back to the place in Star Trek: Deep Space Nine's (DS9) season 2 episode "Crossover." Since then, the gates to the Mirror Universe have stayed busy. Along with subsequent episodes of DS9, the Mirror Universe and its twisted characters have returned in the prequel series Star Trek: Enterprise (ENT) and Star Trek: Discovery, and creators of non-canonical Trek media have taken the opportunity to bring the crews of other series to this weird world. 

But what is the Mirror Universe? To find out, keep reading for Star Trek's Mirror Universe explained.

WARNING! Spoilers for multiple Star Trek series follow!

What is the Mirror Universe?

What if everything was different? What if cowards were heroes, heroes were villains, and villains ruled over everything? Star Trek answers this question with its parallel reality, the Mirror Universe.

These days it's tough to have any kind of interest in science fiction and not be familiar with the concept of parallel universes, but Star Trek's Mirror Universe isn't just different. As the name implies, the people of the Mirror Universe are often the exact opposite of their Prime Universe counterparts. The United Federation of Planets and its hallowed ideals are replaced by the Terran Empire and a barbaric "might makes right" philosophy.  In Starfleet's place is the Imperial Starfleet, and its captains — known for their compassion and tolerance in the Prime Universe — are ruthless, unforgiving, and often xenophobic. The quickest way to get ahead in the Imperial Starfleet is to assassinate your superior officer, and the only consistent forms of discipline are torture — usually in the form of the excruciating agonizer booths — and death.

While the Mirror Universe characters are the opposite of the Prime Universe counterparts, that doesn't always change their roles in terms of being heroes or villains. The Mirror Universe version of DS9's Miles O'Brien, for example, is largely depicted as being heroic, though the Mirror O'Brien is willing to do things — like kidnapping — in order to achieve his goals that the prime O'Brien probably wouldn't stoop to. 

The first crossing

In 1996's Star Trek: First Contact, Zefram Cochrane lays the groundwork for the founding of the Federation when he greets the first Vulcan to visit Earth. But in the two-part ENT episode "In a Mirror, Darkly," the Mirror Cochrane instead murders the Vulcan, paving the way for the oppression and brutality of the Terran Empire. 

Fast forward a hundred years later to the Terran Empire's version of the Enterprise, where Jonathan Archer is only a first officer. Mirror Archer is much more ruthless than his Prime counterpart, and he stages a coup to take over command of the ship. Among other things, we learn the Mirror versions of Dr. Phlox and Malcolm Reed are the inventors of the excruciating agonizer booths and that T'Pol is working with the Vulcans and other oppressed Imperial races in their rebellion.

Unlike most Mirror Universe episodes, none of the main characters from the Prime Universe cross over ... except in Archer's mind. After learning of the other universe, Mirror Archer has maddening visions of the Prime Archer taunting him. The story cleverly crosses over with the TOS episode "The Tholian Web," transplanting the captured USS Defiant not only from the Prime Universe but through time. Archer and his crew capture the Defiant and use its superior technology to cripple the rebellion and to put Archer in the position to take the Imperial throne. In the final moments of the episode, however, Hoshi Sato — who Archer believes to be his loyal concubine — poisons him, taking the throne for herself. 

A Mirror Universe stowaway

For most of Discovery's first season, the eponymous vessel is commanded by Captain Gabriel Lorca, a man whose sensitivity to light — we're told — is a result of his injuries aboard the USS Buran. By the end of the first season, however, we learn Lorca is perhaps the most deceitful captain in the history of Trek's commanding officers. Lorca is, in fact, a native of the Mirror Universe masquerading as his Prime counterpart. His identity as a Terran accounts for his light sensitivity, though this is the first time we learn of this aspect of Terran biology. 

We never learn exactly when the Mirror Lorca crossed over to the Prime Universe, nor do we know the fate of the Prime Lorca. It's speculated he couldn't have survived alone in the Mirror Universe, but we never find out for sure. All we know is that as soon as he learned of Discovery's unique spore drive, Mirror Lorca did everything he could to find himself in the experimental ship's command chair in order to use the drive to bring himself back to the Mirror Universe and continue his rebellion against Emperor Philippa Georgiou. 

At the end of "Into the Forest I Go," Lorca finally puts his plan into action. When the rest of the crew believes they're using the drive to jump to Starbase 46, Lorca secretly diverts the ship to the Mirror Universe. He reignites his rebellion against the Emperor, only to be killed by Georgiou in "What's Past is Prologue." 

The Emperor's new clothes

In the Prime Universe, Philippa Georgiou was the captain of the USS Shenzhou — Michael Burnham's commanding officer who died during the Battle of the Binary Stars. But in the Mirror Universe, Georgiou is the emperor of the Terran Empire. While the Mirror Georgiou seems content to die fighting Lorca's followers, Burnham saves her against her wishes, bringing her back to Discovery and subsequently to the Prime Universe. 

With unmatched combat skills and a devious mind, Mirror Georgiou becomes a great asset to the clandestine operations of Section 31. The official word from Starfleet is that the prime Georgiou somehow miraculously survived the Battle of the Binary Stars. It's agreed that the existence of the Mirror Universe is to be purged from all records because of the concern that in the wake of the devastating war with the Klingons, Federation citizens who learn of the Mirror Universe might attempt to cross over to find counterparts of their dead loved ones. 

While Georgiou resists any attempts by her new Prime allies to make her any less savage, it's clear she holds a special place in her heart for Burnham whose Mirror counterpart is her adopted daughter. Georgiou eventually rejoins the crew of the Discovery and becomes an integral part of their efforts against the rogue A.I. Control. She's also among the heroes when they choose to purposely strand themselves in the distant 32nd century. 

Kirk visits the Mirror Universe

In the opening of "Mirror, Mirror," Captain Kirk, Scotty, Uhura, and Bones are on surface of the Halkan homeworld, having tried and failed to negotiate dilithium mining rights. But somehow, due to a raging ion storm, when the four Federation officers are beamed back to Enterprise, they find themselves aboard the version of the ship in the Mirror Universe instead of their own. Meanwhile, their Mirror counterparts are sent to the Prime Universe where they're soon imprisoned. "Mirror, Mirror" takes place approximately a decade after Discovery's crossover, but because of Starfleet's decision to keep the place a secret, Kirk and his crew don't know what they're in for. 

On board the Mirror Enterprise, everyone's in strange uniforms, including female crew members being in much more revealing outfits (and it isn't like the Prime Uniforms were particularly conservative). Most conspicuously, everyone gives each other salutes reminiscent of the infamous Nazi one-arm salute. 

Kirk and his colleagues from the Prime Universe struggle to remain incognito while they search for a way back home. See, rather than negotiating, the Terrans are threatening the Halkans with annihilation if they don't allow them to mine dilithium. And Kirk raises suspicions when he refuses to fire on the planet, inspiring an assassination attempt. The bearded Mirror Spock eventually discovers the identity of the Prime Universe heroes and allows them to return, wanting his own captain back. However, Spock's time with the Prime Kirk proves more consequential than anyone at the time imagines.

The fall of the Terran Empire

In season 2 of DS9, heroes from Trek's Prime Universe find themselves in the Mirror Universe for the first time in over a century. In "Crossover," Major Kira and Dr. Bashir come across a very different Deep Space Nine. In the Mirror Universe, DS9 is still named Terok Nor, but Odo is a slavedriver, Sisko is a pirate, and the Ferengi bartender Quark doesn't even know what latinum is.

Rather than Starfleet, Terok Nor is ruled by the Klingon-Cardassian Alliance (KCA). Bajor is a part of the KCA, and the world's ruler — who also presides over the station — is the cold-blooded and seductive Intendant Kira, one of Star Trek's best villains. From the Intendant, Prime Kira learns that Kirk's trip to the Mirror Universe in TOS had far-reaching consequences. Kirk so inspired Mirror Spock that the Vulcan helped bring about a series of compassionate reforms throughout the Terran Empire. Sadly, their lapsed focus on defense made them easier targets for the KCA. The Terrans are now a conquered people, working as slaves for the KCA and forced to wear clothing bearing the symbol of Earth to identify themselves. Because of this, while Intendant Kira treats her Prime counterpart as a treasured pet, Bashir is pressed into slavery as soon as he arrives. 

Kira and Bashir eventually escape with the help of the Mirror versions of Miles O'Brien — better known as "Smiley" in the Mirror Universe — and Sisko. Like Kirk before them, Bashir and Kira inspire change in the darker universe.  

The Terran Rebellion

DS9's lead, Captain Sisko, is kidnapped and brought to the Mirror Universe by Smiley in season 3's "Through the Looking Glass". He learns that when his Mirror counterpart betrayed the KCA in "Crossover," the act started a Terran rebellion. Now, Smiley needs Sisko to help the rebellion by pretending to be his Mirror counterpart long enough for one last mission.

DS9 tracks the conflict between the Terrans and the KCA intermittently throughout the series, offering one Mirror episode every season except for its first and fifth. While the group refers to itself as the Terran Rebellion, other oppressed races are part of the struggle. Star Trek: Voyager's Tuvok — a Vulcan — makes a cameo as part of the rebellion in "Through the Looking Glass." Plus, other races are spotted in their gatherings. Like the Maquis of the Prime Universe, they hide in the Badlands, where the KCA's sensors can't find them. Their leaders include much tougher and no-nonsense versions of Bashir and Jadzia Dax.

Things are looking up for the Terrans by the final DS9 Mirror episode, "The Emperor's New Cloak." By the end of the episode, the Terrans have their own version of the Defiant, complete with a game-changing cloaking device, and they've captured Terok Nor. At the same time, the KCA leadership is in disarray. Intendant Kira is on the run, Garak is dead, and the KCA regent — a much more sadistic and violent version of Worf — is the Terrans' prisoner.

The Mirror Universe rebooted?

Discovery's season 3 two-parter "Terra Firma" reveals the crew's Mirror Universe native Philippa Georgiou is dying because of their journey to the 32nd century. Because 900 years have passed since Georgiou's crossing to the Prime Universe, the Prime and Mirror Universes are no longer aligned, causing her molecular cohesion to deteriorate. 

Learning of a world with a possible cure, Discovery brings Georgiou to a planet where a mysterious man keeps vigil over what appears to be a door leading to nowhere. In the second part of "Terra Firma," we learn this is the Guardian of Forever, who first appears in the classic TOS episode "The City on the Edge of Forever," one of Star Trek's saddest tales. The Guardian allows Georgiou to rejoin her life as emperor of the Terran Empire, where she tries and fails to redeem the Mirror Michael Burnham and save her life. Her attempts at peace and her compassion toward the Mirror Saru, however, convince the Guardian she's worth saving. The Guardian then sends her back to the 23rd century. 

The Guardian makes it clear Georgiou's trip back in time — which ends in her death — was no illusion. Since her Mirror Universe adventure all takes place before the Mirror episodes of Discovery's first season, does that mean most of the history of the Mirror Universe has been altered? Or did her actions create another parallel timeline — a second Mirror Universe?

Time travel — it's confusing and causes problems. 

The untold story of Tiberius

In TOS' "Mirror, Mirror," Prime Kirk discovers his Mirror counterpart has been secretly using a device called the Tantalus Field. With it, Mirror Kirk can monitor anyone on his ship and, should he choose, disintegrate them with the touch of a button. His concubine, Marlena, says it's how he became captain, implying he used it to murder his predecessor. But according to the "Before Her Time" documentary on the season 4 Enterprise Blu-ray, there was almost a reveal that the device did something else entirely. 

According to the documentary, rather than the two-parter which ultimately became "In a Mirror, Darkly," the Enterprise creators wanted William Shatner to reprise his role as the Mirror Kirk, aka Tiberius. In "Mirror, Mirror," it's said Tiberius got the Tantalus Field from an alien scientist's "plundered lab." The proposed Enterprise story would reveal that rather than killing, the device actually sends its victims to a penal colony in the Prime Universe. Tiberius would find himself at this prison, presumably surrounded by a bunch of people he sent there, such as his predecessor, who would likely have been the Mirror Captain Pike. 

Unfortunately, negotiations between Shatner and Paramount didn't work out. However, it's fun to consider the idea of Shatner reprising the role, particularly when you consider how little we see of Tiberius in "Mirror, Mirror." 

Picard's Enterprise in the Mirror Universe

Unfortunately, Star Trek: The Next Generation doesn't include any trips to the Mirror Universe. Plus, the history of the Mirror Universe as it's presented in DS9 would seem to discount any Mirror versions of either the TNG or Voyager crews since the Terran Empire would've been conquered long before those groups existed. But while it's not considered part of official Trek canon, some clever comic book creators found a way to tell the story of the Mirror TNG heroes.

In 2017, IDW released the miniseries Star Trek: The Next Generation — Mirror Broken, written by Scott and David Tipton and with art by J.K. Woodward. We learn that the news of the Terran Empire's demise is the result of exaggeration and rumor. The Klingon-Cardassian Alliance has bitten huge chunks out of the Empire, but it's far from dead. Mirror Broken begins with the Mirror Jean-Luc Picard as captain of the Stargazer, while most of the series follows his efforts to take over the Enterprise. The motley cast includes a bearded Picard, a spiky-haired Wesley Crusher, and a surprisingly confident and competent Lieutenant Barclay.

Broken Mirror isn't the last we see of the ISS Enterprise, either. In 2018, the same creative team released two follow-up miniseries – Through the Mirror and Terra Incognita — that depict the pirates making their way to the Prime Universe.

The Pirate Queen of the Delta Quadrant

The TNG crew aren't the only ones to get the Mirror Universe treatment in the comics. In 2019, IDW released Star Trek: Voyager — Mirrors and Smoke by Paul Allor and J.K. Woodward. A quick prologue explains that Captain Janeway and her crew were part of the Terran Rebellion against the Klingon-Cardassian Alliance before being flung to the Delta Quadrant. Unlike their Prime Universe counterparts, they're quite happy being far from the KCA and decide to stay where they are. 

Voyager's bloody reputation has earned Janeway the title Pirate Queen of the Delta Quadrant. Mirror Neelix and Kes — Delta Quadrant natives who join the crew in the series — are Janeway's rivals. The ship's command structure is a bit different, as well. In the series, Commander Cavit is replaced by Chakotay as first officer after Cavit is killed, whereas in Mirrors and Smoke, it's Chakotay who murders Cavit in order to climb the command ladder.  

One of the most interesting transformations we see is that of Annika Hansen, aka Seven of Nine in the Prime Universe. Hansen was never assimilated by the Borg in the Mirror Universe, though ironically, she still plots — alongside the Doctor, who's disgruntled from his poor treatment by the Terrans — to take over the ship.