Mirror Universe

The Mirror Universe is the setting of several narratives in the Star Trek science fiction franchise, a parallel universe existing alongside, but separate from, the fictional universe that is the main setting of Star Trek. It resembles the main Star Trek universe, but is populated by more violent and opportunistic doubles of its people. The Mirror Universe has been visited in one episode of Star Trek: The Original Series, five episodes of Star Trek: Deep Space Nine, a two-part episode of Star Trek: Enterprise, a storyline woven through the first season of Star Trek: Discovery (continuing with a Mirror Universe character in the main cast of seasons 2 and 3), and several non-canon Star Trek tie-in works. It is named after "Mirror, Mirror", the Original Series episode in which it first appeared.

The characters in the Mirror Universe are aggressive, mistrustful and opportunistic in personality. Whereas the Star Trek universe depicts an optimistic future in which the Earth-based United Federation of Planets values peace, co-operation and exploration, episodes set in the Mirror Universe feature the human-dominated authoritarian Terran Empire which values war, despotism and conquest instead. Humans in the Mirror Universe are typically referred to as "Terrans".

In Star Trek: Discovery, it is stated that humans from the Mirror Universe suffer from photophobia (a sensitivity to light).

The Mirror Universe was first introduced in the original Star Trek episode "Mirror, Mirror", which featured a brutal Empire, managed by humans and their Vulcan allies, in place of the United Federation of Planets. The Mirror Captain Kirk of the ISS Enterprise was a mass murderer who was promoted to Captain after assassinating Captain Christopher Pike. Discipline aboard starships was enforced through agony booths and agonizers carried by crewmembers. Officers were barbaric in behavior and advanced in rank by killing superiors who they thought were incompetent. Nazi-style salutes were used by crewmembers to show loyalty to their captain. Spock's double wore a goatee, which became a popular culture trope as a visual marker for an evil version of a character.

The Mirror Universe was later revisited in the Deep Space Nine second-season episode "Crossover"; it was turned into a story arc that spanned into the final season, with five Mirror Universe episodes over the course of five seasons. The series reveals that when exposed to individuals from the normal universe, the Terran Empire began to reform itself for the better, but was overthrown in the 23rd century by an alliance of alien species who took advantage of the Empire's self-weakening and conquered it, enslaving humans and Vulcans in the process.

A two-part episode of Star Trek: Enterprise, entitled "In a Mirror, Darkly", introduces the early developments of the Mirror Universe. This is the first episode which takes place entirely in the Mirror Universe, and in which no Prime Universe characters appear.

The first season of Star Trek: Discovery has a storyline involving the Mirror Universe. Captain Gabriel Lorca, commander of the USS Discovery, is discovered to be an inhabitant of the Mirror Universe on account of his intolerance to bright light, a genetic trait common to all humans from the Mirror Universe.

In 2018, Comic Book Resources rated Discovery's Mirror Universe saga as the 18th best multi-part episode story of Star Trek.