Discovery in the mirror universe and the fall of Lorca

In 2257 a spore-drive jump went wrong and threw the USS Discovery into the mirror universe, a parallel reality ruled by the brutal Terran Empire. To survive, the crew posed as their own counterparts, while Michael Burnham infiltrated the Empire in the role of her mirror self, a feared captain presumed dead.

The journey revealed a long-hidden truth: Captain Gabriel Lorca was not the Starfleet officer he claimed to be, but the mirror Lorca, who had crossed into the prime universe and manipulated events to get Discovery to take him home. Once there, he launched a coup against Emperor Philippa Georgiou, the mirror counterpart of Burnham's late captain, aboard the imperial flagship ISS Charon. Burnham sided with Georgiou, and Lorca was killed in the fighting.

The Charon was powered by a reactor that drew on the mycelial network, and its damage threatened all life in every universe. Discovery destroyed it, then made its way home, but returned to find that months had passed. Burnham secretly brought the deposed Emperor with her, a decision that shaped the end of the Klingon war and, later, the creation of the Section 31 storylines.