[Originally aired January 7, 2018 on CBS All Access]
Unsure of where they are, the Discovery crew are surprised when a Vulcan ship approaches and fires on them. Temporarily stunned, they are hailed by the Cooper, a ship Lorca thought was being refitted, and told to sit back and the Cooper would take care of the "rebel ship."
Lorca realizes that they are no longer in their own dimension and shares his data from the spore jumps with Burnham and his senior staff.
Tilly and Culber discuss Stamets' continued medical condition until Lorca arrives and sends Tilly away. Lorca wants to put Stamets under the care of a different doctor but Culber defies the order, insisting he is the best person to take care of Stamets despite their personal relationship. Lorca insists his emotions will be detrimental to Stamets' treatment and repeats his orders.
Tyler is sent in a shuttle to extract a data core from the wreckage of a destroyed Klingon ship floating in the debris field in order to learn more about this dimension. Tyler suffers more violent flashbacks of his torture while making the extraction but manages to retrieve the data core. Lorca reveals to Burnham that he knows she and Tyler are in a relationship and he advises her to act professionally despite that.
Back on the ship, Tyler confronts L'Rell again and even releases her from her cell in the brig. She activates a sleeper response in him with a Klingon prayer but he re-asserts his control and locks her back into the cell. She is surprised and says that something has gone wrong. He escapes to the mess hall where Burnham finds him and they discuss his condition.
After Burnham decrypts the data bank, she discovers that this dimension is ruled by the Terran Empire, a human-only dictatorship let by the faceless Emperor and that the non-Earth species - the Klingons, Andorians, and Vulcans - have united to rebel against the Terrans. When the Discovery is hailed again by the Cooper, they have to scramble to avoid being attacked. Reviewing the crew manifest of this universe's Discovery, Burnham realizes Lorca is not the captain of the ship. Tilly is.
Lorca and Burnham formulate a plan once they find out that there are records of at least one ship, the U.S.S. Defiant, coming through from their dimension. Burnham will pose as her mirror universe self (the captain of the Shenzhou) and return to the ship, having taken Lorca (an infamous fugitive in the mirror universe) prisoner. Once aboard, she will get the information about the Defiant and they will return to the Discovery. Tyler is assigned to be her personal guard.
Tyler goes to Culber to ask he look more closely at him for potential problems from his time aboard the prison ship. Culber discovers that Tyler's scarring and wounds are not just the souvenirs of torture: it appears the Klingons actually modified his body altogether. Furthermore, although they tested him when he came aboard for buried brain-washing commands, he suspects that Tyler's exterior personality is actually what the Klingons created as an overlay for a different personality, the original personality. Tyler refuses to believe this and when Culber tells him he isn't who he thinks he is, he snaps the doctor's neck.
Finding the Shenzhou, the Discovery makes contact with Captain Tilly in command, having undergone a physical makeover and quick-study on her mirror-universe persona. Burnham, Lorca, and Tyler beam over and Burnham takes over command from Connor, whom she'd last seen in her home universe, being sucked into space.
On board the Shenzhou, they deliver Lorca to the brig where prisoners are held in mechanized torture chambers called agonizing booths. On the way to her ready room, Connor tries to kill Burnham and she kills him in self-defense, arriving on the bridge with his corpse, to the delight of her bridge crew.
When she returns to her quarters after shift, she and Tyler sort of declare their love for each other and share a romantic interlude while Lorca is tortured in his cell.