Despite Yourself

Discoverys crew determines that they have arrived in a parallel Mirror Universe, with Stamets now unconscious and unable to power the spore drive. Tyler confronts L'Rell, and she attempts to use a verbal cue to trigger something within him, though he fights this off. They find a data core in the wreckage of a Klingon ship and learn that this universe is ruled by the human Terran Empire, who are fighting a resistance that includes species such as Klingons and Vulcans. Here, Burnham is the former captain of the ISS Shenzhou, presumed dead after an attack by the fugitive Lorca. The ISS Discovery is captained by Sylvia Tilly's counterpart, so Tilly and the crew pretend to be their Mirror selves. They deliver Burnham and Lorca to the Shenzhou, under the ruse that Burnham had been secretly hunting Lorca since her presumed death and has now captured him.

Summary from Wikipedia, CC BY-SA 4.0.