The ISS Enterprise is the flagship of the Galatic Empire. It was originally seen in The Original Series (TOS) of
Star Trek in the fourth episode of the second season "Mirror Mirror" which first aired on 10.June.1967.
The ISS Enterprise exists in an alternative universe where there is no Federation of planets and instead Earth has expanded an Empire across the galaxy.
In the Empire promotions are obtained through murder of superiors, insubordinate crewmembers were openly punished via several methods of torture, and violence was openly advocated as Starfleet policy.
The ship is an exact replica of the USS Enterprise all crew members are almost exactly the same but with evil personalities and some small physical differences.
Among the most noticeable is a scar on Sulus cheek and Spock sporting a rather fetching goatee. A Californian band has even named themselves after this fact Spock's Beard
The episode consists of members of the USS Enterprise (Kirk, McCoy and Uhura) switching places with their evil counterparts and their attempts to return to their own universe.
Bibliography: www.startrek.com