Over a hundred years ago, when he was a young man on his first Five Year mission as Captain, James T. Kirk found himself in a universe that was a Mirror of our own - where he and his crew were the evil envoys of the Terran Empire, imposing their cruel will on the helpless people's of the galaxy. On Kirk's advice, the Spock of that universe brought that Empire down.
Now, a hundred years later, the Terran Empire is gone and humans live as slaves of the Klingons, Cardassians, and Bajorans. A galaxy of enslaved humans is not a legacy James T. Kirk find acceptable. Somehow he must begin to put things right for an entire galaxy - with the mirror Spock that is not his old friend by side, and the Mirror Universe Captain Picard and Janeway armed against him.