Summary

• Nearly 60 years ago, Star Trek: The Original Series introduced the Mirror Universe.
• Star Trek: Discovery returned to that parallel dimension in Season 1, showing audiences more of that world.
• In the final season of Star Trek: Discovery, the fate of a key vessel in the mirror universe is revealed.

The following contains spoilers from Star Trek: Discovery, Season 5, Episode 5, "Mirrors," now streaming on Paramount+.

One of the most interesting concepts in the Star Trek mythology is its "mirror universe," a parallel dimension where almost everyone is evil. Of the universe's dozen series, only four ever explored it, and only two did so more than once. Star Trek: Discovery made the mirror universe key to its first season, and in its final one, the series revealed what happened to the ISS Enterprise 57 years after it was first introduced. When the show first debuted, some long-time fans felt the crew and the captain were not in keeping with the tenor of proper Starfleet officers. However, Season 1 revealed Captain Lorca hailed from that universe, explaining why he seemed more "evil" than the typical starship captain.

Once the USS Discovery traveled through a wormhole to the 32nd Century, Dr. Kovich told Dr. Culber that the Mirror Universe and Prime Universe were too far apart for "crossings" to occur any longer. However, while on the hunt for the clues to the location of the Progenitors' technology, Captain Burnham and Cleveland "Book" Booker find a pocket of interdimensional space housing a vessel from the Mirror Universe. However, it's not any old starship, it is the ISS Enterprise last seen in "Mirror, Mirror" when the Prime Universe's Captain Kirk told Mirror Universe Spock it only took one good man to start a revolution. Once Star Trek: Deep Space Nine reintroduced the Mirror Universe, what happened to the ship was an open question Discovery just answered.

Kirk: What worries me is the easy way his counterpart fitted into that other universe. I always thought Spock was a bit of a pirate at heart.

Spock: Indeed, gentlemen. May I point out that I had an opportunity to observe your counterparts here quite closely. They were brutal, savage, unprincipled, uncivilized, treacherous - in every way splendid examples of homo sapiens, the very flower of humanity. I found them quite refreshing. -- Star Trek: The Original Series "Mirror, Mirror"

Where the Mirror Universe and the ISS Enterprise Came From

When Gene Roddenberry put together his pitch for Star Trek 60 years ago in 1964, a loose idea of the Mirror Universe concept was on an early pitch document. The full concept came from writer Jerome Bixby, based on his decade-old short story "One Way Street." The writer said "the universe [he] created was a very savage counterpart" and that "it's arguable...the universe itself might be termed a 'character,'" in The Captain's Logs Supplemental by Edward Gross and Mark A. Altman.

In "Mirror, Mirror," Kirk, Doctor McCoy, Uhura and Scotty are sent via transporter accident to the Mirror Universe. The episode is mostly about them trying to survive long enough to be returned to their own universe. However, Mirror Spock figures out the captain and crew aren't from his version of reality. So, he ends up helping them recreate the accident so they can return home. Tiberius Kirk and the evil versions of the crew are also sent back, but it's strongly implied that Spock will quickly take control from him.

Star Trek: The Original Series "Mirror, Mirror" Official Synopsis: A transporter accident places Captain Kirk's landing party in an alternate universe, where the Enterprise is in the service of a barbarically brutal empire.

In Star Trek: Enterprise, a two-part episode set in the Mirror Universe brought the USS Defiant from The Original Series episode "The Tholian Web" to the past. This helps explain why the ISS Enterprise is so much like the Constitution class vessels from the prime universe. Each starship is also equipped with a Tantalus Field, a mysterious device that makes a captain's enemies vanish. Presumably, Spock used the vessel to start his revolution, and Deep Space Nine reveals how it all turned out. Yet, what happened to the ISS Enterprise remained a mystery, until "Mirrors."

The USS Discovery's Search for Clues Led Burnham to the Enterprise

Captain Burnham and the USS Discovery crew are familiar with the USS Enterprise, though Captain Christopher Pike was her commanding officer then. After surviving the Time Bug placed on the ship by Moll, Burnham and Book take a shuttle into an "aperture" of extradimensional space to follow their ship's warp trail. When they enter it, Burnham recognizes the ship, but tells Book that during her time in the Mirror Universe in Season 1, she never saw that particular vessel. The ship has been stuck there for some time, and it's damaged. Not just from the pocket dimension it sits in, but it had been in a battle.

Star Trek: Discovery "Mirrors" Official Synopsis: Captain Burnham and Book journey into extradimensional space in search of the next clue to the location of the Progenitors' power, while Rayner navigates his first mission in command of the U. S. S. Discovery and Culber opens up to Tilly.

When Burnham and Book board the vessel, they discover something surprising. Rather than a Terran warship, they see the ISS Enterprise seemingly serving as a home to refugees. After restoring some power to the ship, they are able to locate Moll, L'ak and the clue they seek by scanning for the quantum signature of people and objects from the prime universe. A short fight breaks out, but circumstances align so that Moll and Booker have to work together to free L'ak and Michael from a security protocol.

When L'ak and Michael fight, she's able to get the clue and she, accidentally, wounds the Breen exile. Moll and L'ak escape in the shuttle Michael and Book arrived on, so they have to figure out a way to get the ISS Enterprise out of the extradimensional space it was marooned in so long ago. Naturally, they succeed with the help of quick-thinking by Commander Rayner on the USS Discovery. What's most interesting, however, is that throughout this adventure no one really wonders just how the clue from the Prime Universe ended up on the flagship of the Terran Empire.

Deep Space Nine Revealed the Fate of Spock's Terran Empire

The first show to return to the Mirror Universe was the first serialized Star Trek, Deep Space Nine in Season 2, Episode 23, "Crossover." Naturally, because that series was set on the space station close to the planet Bajor, this was the corner of the Mirror Universe that the episode (and its many sequels over seven seasons) explored. Rather than the jingoistic rules of the Terran Empire, however, humans were an oppressed class working the ore processing facilities on the Deep Space 9 station. It was ruled by an alliance of Klingons, Cardassians and Bajorans.

Star Trek: Deep Space Nine "Crossover" Official Synopsis: Kira and Dr. Bashir are accidentally sent to the Mirror Universe and discover that it is dominated by a ruthless Klingon–Cardassian alliance and Terrans (humans) are slaves.

Kira Nerys met her counterpart, the leader of the station known as "the Intendant." She explained to her what happened after the real Captain Kirk transported back to his universe. Spock used the ISS Enterprise to become Chancellor of the former Empire, all the while making institutional reformations that made the society more peaceful and equitable. However, after years of being oppressed by the Terrans, the Klingon and Cardassian alliance was able to launch a successful campaign against them.

The Klingon-Cardassian Alliance is the dominant power in the Alpha Quadrant and controls the space near Bajor in the Mirror Universe. The alliance is mostly benevolent and a lover of liberty, but the realities of dealing with a foe like the Terran Empire forces them to engage in questionable tactics.

Still, Deep Space Nine left the ultimate fate of the Terran Empire -- and, more specifically, the ISS Enterprise -- an open question. In fact, Intendant Kira never really clarified when the Terran Empire fell, beyond saying that Spock's reforms began "almost a century ago." Still, knowing that the Terran Empire fell under a brutal assault by Klingons and Cardassians, a picture starts to emerge about why the ISS Enterprise would have been a home for refugees and made the perilous, impossible journey across dimensions.

A Terran Refugee Hid the Clue In the ISS Enterprise In the 24th Century

At the end of "Mirrors," Michael Burnham reveals the scientist who hid the clue on the ISS Enterprise was able to do so because she was, herself, a Terran. Some time before The Next Generation's "The Chase," she and a group of refugees fled the ISS Enterprise in shuttles and made their way into the Prime Universe. From there, many of them made homes in the Federation, and this particular scientist was the Junior Science Officer on the ISS Enterprise, Dr. Cho.

Star Trek: The Next Generation "The Chase" Official Synopsis: The crew of the Enterprise must race against various rival powers to uncover an archaeological secret that explains the predominance of humanoid life forms in the galaxy.

This means the Terran refugees fled the Mirror Universe sometime before the events of "The Chase." Yet, it was close enough to those events that Dr. Cho was able to make her way back to the ship that carried her and her fellow immigrant in order to hide the clue there. As Booker notes in the episode, the clues to the Progenitors' technology each come with a lesson. In this case, Dr. Cho wanted to subtly teach the searchers who followed her that things can always get better. She went from being a Terran scientist to a Branch Admiral in Starfleet. It's kind of like Starfleet's first mutineer becoming the captain of the first vessel on which she served afterward.

No matter the timeline or universe, the Enterprise is an important, historic vessel. The ISS Enterprise was a warship that brought fear and terror to whomever it visited. At least, until a man named Kirk told a man named Spock there was a better choice to make. In its final season, Star Trek: Discovery has found yet another way to tie its story into the fabric of the universe's past and the message Gene Roddenberry and all those who followed him wanted to give the audience. Things can always be better, but it takes people making the right choices to get there.

Star Trek: Discovery debuts new episodes Thursdays on Paramount+.