This sequel to the TOS episode "Mirror Mirror" picks up the story in the alternate universe immediately after. Spock plans to rebel against the Empire in a quest for peace, but first must plot against Kirk to take control of the Enterprise.

FAN NOTE: This is a follow up episode to "Mirror, Mirror" (s2e04) from the original series.

Captain Kirk finds himself haunted by guilt from his past as the fate of an alien world hangs in the balance.

FAN NOTE: This episode involves original series character story lines: Rayna from "Requiem for Methuselah" (s3e19), Miramanee from "The Paradise Syndrome" (s3e03), and Edith Keeler from "The City on the Edge of Forever" (s1e28).

Kirk has to choose between Spock and a competing female officer for captaincy of another starship, but issues of competence in the role as well as gender issues cause difficulties.

To solve the utmost mystery, the Enterprise must return to where Kirk's five-year mission began.

FAN NOTE: This is a follow up episode to "Where No Man Has Gone Before" (s1e03), the second pilot for the original series. This episode has characters called "espers". In the original series pilot, Mitchell said, "Man cannot survive if a race of true espers is born."

FAN NOTE: The Romulan Commander character was introduced in the original series episode "The Enterprise Incident" (s3e02). The actress who played that role in the original series is the mother of the actress playing the Romulan Commander in this episode.

The iconic mission of the U.S.S. Enterprise comes to an end, as Kirk and his crew battle the ultimate adversary.