Created by Gene Roddenberry in 1966, the space adventure franchise Star Trek has endured for generations, spawning dozens of series, movies, and video games, with more related projects in the works. It has featured breezy and comedic episodes, but has also explored sensitive topics like fundamentalism, cloning, mental health, addiction, xenophobia, A.I. control, and bioweapons.

As a staple of science-fiction television, Trek shows have naturally tackled time travel, supernatural beings, noncorporeal lifeforms, space anomalies, and alternative or parallel universes. One of the latter was dubbed the Mirror Universe by fans, and here is how it came to be and what it entails. To clarify, when referring to this particular type of universe, the main timeline and its characters are labeled “Prime,” while their counterparts are slapped with the “Mirror” designation.

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The Mirror Premise at a Glance

In the Prime timeline, most humans aim to live in harmony with as many contacted alien species as possible; the main objective is building a universe of peace and tolerance, where medical advances can cure almost every ailment and injury, and special abilities and superpowers are celebrated and harnessed instead of feared and condemned. As Captain Picard explains to a man from the past:

The acquisition of wealth is no longer the driving force in our lives. We work to better ourselves and the rest of humanity.

Although populated with the same characters and occupying the same space, the Mirror Universe is set in another dimension, and features twisted or strange doppelgängers. Instead of avoiding mass destruction and preaching acceptance, the ruling Terran Empire is a ruthless, fascist regime that goes to great lengths to oppress and subjugate every species.

It even treats its own people in the same manner, punishing officers who commit “mistakes” with a device called the Agonizer, or putting them in the Agony Booth, and mercilessly eliminating any opponents and obstacles. Sometimes, Prime characters venturing into the Mirror Universe can prove beneficial. In Star Trek: Discovery, for instance, an adventure in the alternate timeline helps put an end to the war with the Klingons.

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Another example is when Prime characters in Star Trek: Deep Space Nine influence evil doppelgängers into a change of conscience, leading to political reforms. Unfortunately, it is later revealed that the Terran Empire is overthrown, only to be replaced by an alliance of Klingons, Cardassians, and Bajorans, who enslave both humans and Vulcans.

It's important to note that Mirror characters are not only often polar opposites personality-wise, but they also have subtle biological differences, such as more sensitive eyesight, and they experience faster cellular decay when they stay in the Prime Universe for too long.

Canon Shows with Mirror Universe Episodes

Before diving into the explanation of the Mirror Universe, here is a breakdown of the four canon series of Star Trek that feature this parallel dimension:

Star Trek: The Original Series (1966-1969)

Honorable Mention: The Tholian Web: Technically not set in the Mirror Universe, it features the vanishing of the USS Defiant into Tholian space, and the connection to a parallel dimension is established later on, in Star Trek: Enterprise’s episode, “In a Mirror, Darkly.”

These are the voyages of the starship Enterprise. Its five-year mission: to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before.

Set in the 23d century, The Original Series (TOS) follows Captain James T. Kirk (William Shatner) and his crew, namely First Officer and Science Officer Spock (Leonard Nimoy), Chief Medical Officer Leonard McCoy (DeForest Kelley), Chief Engineer Scott (James Doohan), and Communications Officer Lt. Nyota Uhura (Nichelle Nichols), as they explore the Milky Way and interact with various lifeforms, some more hostile than others.

Star Trek: Deep Space Nine (1993-1999)

Deep Space 9 (DS9) is set in the 24th century, and centers on the titular space station located at the end of the galaxy, next to a wormhole connecting Federation space to the Gamma Quadrant. Main characters include Captain Benjamin Sisko (Avery Brooks), Science Officer Jadzia Dax (Terry Farrell), Head of Security Odo (René Auberjonois), Chief Medical Officer Julian Bashir (Alexander Siddiq), and First Officer Kira Nerys (Nana Visitor).

Together, they protect the Alpha Quadrant from hostile species like the Cardassians and the ultimate rulers of the Gamma Quadrant, the Changelings, who are fluidic shapeshifters.

Star Trek: Enterprise (2001-2005)

Enterprise (ENT) serves as a 22nd-century prequel to TOS, centering on Captain Jonathan Archer (Scott Bakula) and his crew in the first-ever exploration of the Alpha and Beta quadrants under the flag of the United Earth, aboard the Enterprise NX-01. They save the Earth from the hostile Xindi aliens, establish the guidelines for interstellar diplomacy, and usher in the formation of the United Federation of Planets. It’s basically a prequel to the entire Prime timeline.

Star Trek: Discovery (2017 –)

Discovery (DIS) follows the crew of the titular starship in the 23rd century, a decade before TOS, and later into the 32nd century, after they time travel. Sonequa Martin-Green stars as Michael Burnham, a science officer who starts a war with the Klingon race and then becomes captain of the Discovery. Another key character is her mentor, Philippa Georgio (wonderfully played by Michelle Yeoh), the friendly, honorable, determined, and decorated commanding officer of the USS Shenzhou; she is killed in hand-to-hand combat with a Klingon leader.

The Episode That Ushered in the Narrative

In TOS’ episode “Mirror, Mirror,” the transporter malfunctions during an ion storm and swaps Captain Kirk and the rest of his landing party, McCoy, Scott, and Uhura, with their villainous counterparts from a parallel universe that exists in the same space. This alternate dimension sees the Federation’s USS Enterprise, now called the ISS Enterprise, as a flagship for the evil Terran Empire.

Disoriented at first, Prime team members decide to play along until they’re able to recreate the same conditions that would send them back home. Before leaving, Prime Kirk reveals to Mirror Spock the existence of the Tantalus Field, Mirror Kirk’s secret device that can monitor and eliminate people with a single touch, so that he can use it as he sees fit to reform the Empire.

It is revealed that Mirror Kirk used the deadly device to get rid of his predecessor, Mirror Captain Pike, in order to take over the ISS Enterprise; that alone was shocking to both Prime Kirk and the audience, because it was completely out of character for the Kirk we knew, and cemented the darker aspect of the Mirror Universe as a whole.

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The Tantalus Field makes a comeback in the DIS episode “Terra Firma,” when it is revealed that Mirror Georgiou, the Emperor of the Terran Empire, has been using it to spy on her adopted daughter, Mirror Captain Burnham.

To date, “Mirror, Mirror” is still considered the best Mirror episode in the franchise. It is a well-executed, insightful, and intriguing plot, featuring a particularly ruthless Kirk and an edgier Spock. No wonder it became a defining moment in classic pop culture and inspired fans to come up with the Mirror Universe designation in future related chapters.

Three Fan-Favorite Mirror Characters

TOS’ Mirror Spock and DS9’s Mirror Kira left a particularly memorable impression.

Mirror Spock is a Terran/Vulcan hybrid and First Officer of the ISS Enterprise. Unlike the clean-shaven Prime Spock, who is inclined to self-sacrifice for the greater good, he sports a cool mustache and a goatee and ruthlessly resorts to punishment and torture, even with his subordinates. However, much like his counterpart, he is governed by logic, and thus, Prime Kirk is able to reason with him and influence him for the good of the Empire, and our heroes.

Mirror Spock: Captain, I am pleased that you frustrated Mr. Chekov’s plan. I should regret your death.

Prime Kirk: Why?

Mirror Spock: I do not desire the captaincy. I much prefer my scientific duties, and I am frankly content to be a lesser target.

Prime Kirk: Logical, as always, Mr. Spock.

Fun Fact: Mirror Spock inspired the name for the progressive metal group, Spock’s Beard.

While Prime Kira is a compassionate Bajoran and an honorable Major (and later Colonel) who dresses modestly and fights for injustice, Mirror Kira is the ultimate femme fatale, black leather outfit and whip included. She holds the title of Intendant in the Klingon-Cardassian Alliance and is playful, manipulative, and hedonistic, with a twisted sense of humor.

When she comes face to face with Prime Kira, she is so fascinated by her counterpart that Mirror Kira professes her attraction to Prime Kira: a classic case of narcissism.

Mirror Kira: You know, I bet if we put our heads together, we could create a little excitement for ourselves.

Terran Guard: You sentenced my wife to death.

Mirror Kira: Isn't that a coincidence? I was hoping you weren’t married.

Other noteworthy Mirror characters are Mirror Philippa Georgiou, and Mirror Captain Gabriel Lorca. Georgiou is a complicated villain who starts off as the ruthless tyrannical ruler of the Terran Empire, and ends up in the Prime Universe as a prisoner, working for the Federation, who utilizes her vast skills in war to aid them. Georgiou gets a fascinating redemption arc as a villain through the seasons of Discovery, and her eventual experience of cell degeneration in the Prime Universe leads her to be sent into the past, before the two universes split apart.

And Captain Lorca is the compelling character in the first season of Discovery played incredibly well by Jason Isaacs (Harry Potter, The Patriot), who is hiding in the Prime Universe disguised as his Prime counterpart. He seeks to overthrow Georgiou and rule the Mirror Universe with Burnham at his side, though he was also one of the few Mirror Universe characters who showed a surprising amount of warmth and humanity, in rare moments.

Will there be more Mirror episodes in the franchise? The answer may very well lie in the upcoming seasons of DIS and Star Trek: Strange New Worlds. Perhaps more Prime characters will emerge as their secret Mirror counterparts in disguise, as was the case of DIS' Gabriel Lorca (Jason Isaacs), the power-hungry Captain in the Terran Imperial Starfleet who infiltrated and took command of the USS Discovery in the main timeline.

Meanwhile, all eyes are on the much-anticipated movie, Star Trek: Section 31; it will be directed by Olatunde Osunsanmi, with Michelle Yeoh reprising her role of Philippa Georgiou as a member of the shady titular intelligence organization.