Three Star Trek ships have crossed between the Mirror Universe and the Prime Universe. Introduced in the classic episode of Star Trek: The Original Series, “Mirror, Mirror,” the Mirror Universe is a dark parallel reality featuring evil versions of Star Trek’s characters. Although the Mirror Universe has popped up on several StarTrek series since its introduction, starships rarely make the jump between universes. In “Mirror, Mirror,” it was a transporter malfunction that caused Captain James T. Kirk (William Shatner) and his USS Enterprise crew members to end up in the Mirror Universe, but that’s not the only way characters have traveled between the two realities.

Star Trek: Enterprise revisited the Mirror Universe with season 4’s two-part episode, “In a Mirror, Darkly,” which saw the return of the original USS Defiant. Star Trek: Discovery‘s characters aboard the USS Discovery also visited the Mirror Universe in the show’s first season. In Star Trek: Discovery season 5, episode 5, “Mirrors,” the ISS Enterprise made its first appearance since its introduction in Star Trek: The Original Series. In the Mirror Universe, the authoritarian Terran Empire rose instead of the United Federation of Planets, and Starfleet is comprised of warships like ISS Enterprise that conquer and subjugate other worlds. Here are 3 starships that crossed over between Star Trek‘s Prime and Mirror Universes.

The USS Defiant in Star Trek: Enterprise

Star Trek: The Original Series Season 3, Episode 9 – “The Tholian Web” & Star Trek: Enterprise Season 4, Episodes 18 & 19 – “In a Mirror, Darkly”

Although Star Trek: Deep Space Nine’s USS Defiant would become the more famous ship to bear the name, there have been multiple ships named Defiant, including a 23rd-century Constitution Class starship first seen in Star Trek: The Original Series. In the TOS episode, “The Tholian Web,” the USS Enterprise was sent to locate the Defiant, which they found trapped between universes, with its entire crew dead due to apparent insanity and mutiny. In a conflict with the Tholians, the Defiant was pushed through a spatial hole to an unknown destination.

The final destination of the USS Defiant was revealed in Star Trek: Enterprise’s Mirror Universe two-parter, “In a Mirror, Darkly.” The Defiant both traveled back in time and crossed into the Mirror Universe. The Mirror Universe Tholians then began to salvage the ship, but Commander Jonathan Archer (Scott Bakula) of the ISS Enterprise NX-001 plotted to steal the Defiant and use its advanced 23rd technology to install himself as Emperor of the Terran Empire. Archer commandeered the Defiant but en route to Earth, Lt. Hoshi Sato (Linda Park) poisoned Archer and took command of the Defiant, declaring herself Empress.

The USS Discovery in Star Trek: Discovery

Star Trek: Discovery Season 1

Star Trek: Discovery season 1 spent five episodes in the Mirror Universe after Captain Gabriel Lorca (Jason Isaacs) manipulated the starship and crew back to the alternate reality he originated from. Previously unknown to the Discovery’s crew, Lorca was actually from the Mirror Universe and intended to use Michael Burnham (Sonequa Martin-Green) to gain access to Emperor Philippa Georgiou (Michelle Yeoh) and her ship. Lorca altered the coordinates for Discovery’s spore drive, knowingly taking the ship into the Mirror Universe.

Once in the Mirror Universe, Discovery’s crew had to masquerade as their evil counterparts as they searched for a way to get home. Burnham eventually discovered Lorca’s ruse and thwarted his plan before Emperor Georgiou ultimately killed him. Lorca’s betrayal deeply affected Burnham and the rest of Discovery’s crew, and their time in the Mirror Universe greatly cost the Federation. When Discovery returned to the Prime Universe nine months after they had left, the Federation/Klingon War was going badly, but Burnham and Georgiou helped bring the conflict to an end.

The ISS Enterprise in Star Trek: Discovery

Star Trek: Discovery Season 5, Episode 5 – “Mirrors”

Star Trek: Discovery season 5 sends Captain Michael Burnham and the USS Discovery on an intergalactic treasure hunt for the powerful technology of the Progenitors, the creators of humanoid life. As part of their search, the crew of the Discovery finds a wormhole that leads to a pocket of interdimensional space, where they find the abandoned ISS Enterprise. The 23rd-century starship became a lifeboat for refugees fleeing the brutal Mirror Universe. According to a plaque on the ship, Mirror Universe Spock (Leonard Nimoy) implemented a series of reforms for a more peaceful way of life after his encounter with Prime Universe Captain Kirk (William Shatner).

Although Spock was killed for his beliefs, he inspired others to set out in search of a better life. Despite an undoubtedly difficult journey, at least some of the refugees from the Mirror Universe’s ISS Enterprise made a life for themselves in the Prime Universe. One of the refugees, a woman named Dr. Cho, became one of the seven scientists who helped hide the Progenitors’ technology. She placed her clue on the ISS Enterprise, which remained undisturbed for centuries. Star Trek: Discovery season 5 has revisited several storylines from Star Trek’s history, and the return of the ISS Enterprise serves as a nice shout-out to the Star Trek series where it all began.