The mirror universe is a parallel universe where the patterns of events move in similar manners to that of the primary universe, but the intentions and characterizations are different, often showing "evil" characteristics in place of "good" and vice versa. Office of Naval Intelligence, by 2267, referred to this reality as "Terran Empire parallel." The mirror universe shares a similar history to that of the primary universe and many of the same people exist in this reality, but they and the galaxy they live in are skewed versions of their "normal" counterparts.
It is not clear if the mirror universe diverged from the primary universe at some point or always existed in tandem, mirroring events and peoples. If the two universes did diverge it was at a point many hundreds of years ago. Phlox noted that the "great works" of literature in both universes were roughly the same except that the primary universe characters were "weak and compassionate," with the exception of that William Shakespeare and his counterpart.
On 5 April 2063, first contact between Vulcans and Terrans took place much as it did in the primary universe. However, once the Vulcans landed, instead of welcoming them with open arms, Zefram Cochrane shot the first Vulcan to step onto Terran soil and the Terrans proceeded to raid the Vulcan ship, the T'Plana-Hath. The gun used by Cochrane later would come into the possession of Commander Jonathan Archer, who wondered what would have happened had Cochrane not "turned the tables on [the Vulcans'] invasion force." With advanced Vulcan technology now at their disposal, the Terrans were able to expand the Terran Empire and conquer other worlds, including Vulcan.
- It is not clear when the Empire began. Archer once stated that the Empire had existed for "centuries" as of 2155. Archer did not mention how many centuries, but by his statement, the Empire can be traced back to at least 1955, suggesting that it was a Terran political unit before it became an interstellar empire. The credits sequence for In a Mirror, Darkly used footage of battles going back at least to the "Age of Sail". It also appears that it is the empire that conflicts with the Xindi.
The Bajorans and Trill had empires until they were conquered by Earth.
Hundreds of years prior to the 24th Century, Kai Dava Nikende foresaw the conquest of Bajor by the Terran Empire in 2255 and the suppression of the Bajoran religion which involved the killing of its priests and the destruction of its icons such as the Orbs of the Prophets. Consequently, he preserved a fragment, known as a paghvaram, from each of them. During his orb experience with the Orb of Souls, he made contact with his primary universe counterpart, who was likewise the Kai of Bajor, and one of the fragments was transported to that universe for safekeeping. This may have been the first direct contact between the two universes. The paghvaram was kept in the village of Sidau in Hedrikspool Province until it was stolen by Iliana Ghemor in 2376.
In 2155, the ISS Enterprise, under the command of Captain Maximilian Forrest, was the flagship of the Imperial Starfleet of Terran Empire. In January of that year, Commander Jonathan Archer mutinied against Forrest in order to take the ship into Tholian space to capture the primary universe starship USS Defiant which the Tholians had obtained. While the mutiny was ultimately unsuccessful, Captain Forrest had no choice but to continue the mission, since the ship's helm had been locked on auto-pilot. The Enterprise was later destroyed by Tholian ships, but not before Archer was able to take control of the Defiant. Archer tried to use the Defiant in a grab for power, intending to replace the Emperor, but was betrayed and killed by Hoshi Sato, who declared herself Empress.
In possession of the USS Defiant the mirror universe was aware of the primary universe.
However it was not until 2267 that any official contact between the universes is recorded. By this time the Terran Empire was the dominant power in the Alpha Quadrant. In a transporter accident caused by interference from an ion storm a landing party from the USS Enterprise switched places with an identical party from the ISS Enterprise in the mirror universe. The four Starfleet officers; Captain James T. Kirk, Lieutenant Uhura, Lieutenant Commander Montgomery Scott and Doctor Leonard McCoy found the Terran vessel to be brutal regime, almost dictatorial in its command structure. Advancement through assassination was commonplace.
During the encounter, Captain James T. Kirk convinced the counterpart of his Executive Officer, Spock, that the Empire could not sustain itself. Indeed, Spock predicted that in its current form, the Empire had just over two centuries before total collapse.
Shortly thereafter, Spock rose to become leader of the Terran Empire, proposing a series of reforms designed to make the Empire more secure and less dictatorial in nature. These included a significant disarmament program. Unfortunately, once these reforms were complete, the Empire was unable to defend itself against the equally aggressive and powerful forces surrounding it. The Klingon-Cardassian Alliance overran the Empire, conquering Earth and leaving Terrans and Vulcans enslaved.
- Spock's rise to power is detailed in The Sorrows of Empire.
During this time, the forces of the mirror universe began implementing safeguards to prevent another crossover event with the primary universe. Transporter design was altered to prevent interdimensional travel, requiring the creation of devices specifically for that purpose, including the multidimensional transporter. In the event of another crossover, those involved would be killed to prevent further interference.
In 2293, after being snared in the Janus Prime Vortex, Captain Hikaru Sulu and the crew of the USS Excelsior found themselves in the mirror universe, transposed into the bodies of their duplicates, aboard the ISS Excelsior. In order to return to their own universe, Sulu's crew were forced to use the Imperial starship and its fighters to escape from the Terran Empire, and enlist the aid of the Klingon Alliance and the Romulan Republic in fighting the Tholian Empire to gain access to the Janus Ultima Vortex.
By the early 24th Century, the Terrans had been reduced to a slave race and a handful of rebels on the run from the Klingon-Cardassian Alliance, which was intent on destroying the entire Terran race. One such ships used by the Terran Rebellion was Stargazer, captained by Gilaad Ben Zoma.
By 2370, the second major contact with the primary universe took place. A runabout from station Deep Space 9 entered the mirror universe following an incident in the Bajoran wormhole. Kira Nerys and Julian Bashir were captured by forces from Terok Nor, and interrogated by the commander, Intendant Kira Nerys. Bashir was sent to work in the ore processing plant, where he befriended Miles O'Brien. After instigating a series of incidents aboard the station, including the death of Odo during a slave uprising, Kira and Bashir convinced privateer Benjamin Sisko to rebel against the Alliance and help them to escape back to the primary universe.
A year later, Smiley crossed over to the primary universe and impersonated his counterpart long enough to capture Benjamin Sisko. He took Sisko back to the mirror universe, where he convinced him to impersonate his mirror self who had been killed in a skirmish with Alliance ships. The rebels needed the other Sisko to win over Jennifer Sisko, Sisko's wife, who was working as a scientist for the Alliance. Jennifer had been developing a transpectral sensor array which would have allowed the Alliance to locate rebel hideouts in the Badlands. Fortunately for the rebels Ben was able to convince Jennifer to defect to the rebels.
In the Badlands, Chakotay, a former slave, commanded a ship in the rebellion attempting to evade pursuit. There he encountered a strange ship which had been catapulted seventy thousand light-years across the galaxy.
While the USS Defiant of another reality was travelling between universes in 2371, it discovered Terok Nor and was fired upon by a fleet of Klingon birds-of-prey and Cardassian Galor class warships before escaping to a different reality.
While in the Federation's universe, Smiley downloaded information from Deep Space 9's computers, including the plans for the USS Defiant. In 2372, the rebels constructed their own version of the Defiant, but had trouble getting it to function properly. Jennifer lured Ben Sisko back into the mirror universe to help. By this time the rebellion had grown in strength, culminating in the capture of Terok Nor, which became a rebel base of operations.
In 2374, Intendant Kira sent a thief, Bareil Antos to the primary universe to steal one of the Bajoran Orbs, believing it would permit Kira to unite Bajor under her rule. The attempt failed, and Bareil returned to the mirror universe without the orb.
In 2375, Grand Nagus Zek, the leader of the Ferengi Alliance in the primary universe, used the multidimensional transporter to travel to the mirror universe, hoping to open up business opportunities, however he was captured and held hostage by the Alliance. Intendant Kira made arrangements with Worf, the Regent of the Alliance, to obtain a Klingon cloaking device from the primary universe in exchange for Zek's return.
She sent Ezri Tigan, a Trill mercenary and Kira's lover, to give Quark the ransom demand. Quark and his brother Rom stole the cloaking device from Klingon General Martok's ship and delivered it to Ezri, but decided at the last minute that they couldn't trust her to keep her side of the deal, and accompanied her to the mirror universe. There all three were captured by the Terran rebels, who planned to keep the cloaking device. Ezri's companion Brunt freed them, delivering them to Regent Worf. Aboard Worf's flagship, Quark and Rom were imprisoned along with Zek upon discovering Kira's plan, only to be later rescued by Ezri as revenge against Kira for killing Brunt. Ezri ended up joining the rebel cause. The primary universe Ferengi was allowed to return to the primary universe for having aided the rebels' defeat of Worf, a major victory for the rebellion.
From 2376 onto 2381, after the fall of Terok Nor to the Cardassians, the Terran Rebellion united with the Mackenzie Calhoun and the remnants of the Romulan Empire along with the forces of Memory Omega with Jean Luc Picard as their leader to liberate Earth, where they then proceeded to liberate other core worlds of the Terran Empire. Instigating a war between the Klingons and the Cardassians, the Rebellion was able to sue for peace and create the Galactic Commonwealth, a representative republic that was in the process of rebuilding its member worlds.