Discovery is adrift in unfamiliar space. Captain Lorca calls for a yellow alert while they assess the situation. He’s confused about the debris as there were no reported battles in the area. The ship’s systems seem to be malfunctioning. Saru reports that their location relative to the galactic center is accurate, but everything else is off. A Vulcan cruiser approaches. The Discovery hails. The Vulcans power up their weapons. Discovery powers up shields and goes to red alert just as the Vulcans open fire. The Cooper, a ship that’s supposed to be under refit, comes to Discovery’s aid. They hail Discovery and claim the Vulcans are rebels.
Tilly and Culber tend to Stamets, who is unconscious. They cannot make another jump.
Saru reports that both the Vulcan ship and the Cooper have quantum signatures different from the Discovery’s. Lorca realizes that this means they are in a universe that is not their own.
Lorca tells Burnham and Saru about his theory that the mycelium network could connect to parallel universes. The 133 jumps they made filled in some gaps in the network. If they are in an alternate universe, there’s no way they can get the decloaking algorithm back to Starfleet to defeat the Klingons.
Stamets is in a trance-like state with milky white eyes mumbling something about a palace. He shoves Culber away. Tilly apologizes to Culber for not telling him about Stamets’ side effects. Culber blames Lorca, who arrives in sickbay. Lorca accepts the blame. He has also decided to hand Stamets’ case over to another doctor, citing Culber’s romantic relationship with Stamets as affecting his judgment. Culber questions Lorca’s motives. Lorca stands by his orders.
Tyler takes a ship out to explore the wreckage. He discovers a computer core that they hope will be able to tell them whatever the Klingons on board the destroyed ship knew about this universe. Tyler begins having flashbacks. Lorca notices Tyler having trouble maintaining a steady course. Burnham tries to cover for him, giving him the time he needs to recover. He finds a Vulcan and an Andorian body on board the Klingon ship. Apparently, they were in command. Tyler retrieves the data core.
Tyler visits L’Rell in her cell. Tyler says she can’t hurt him anymore. She wonders why she’d ever want to hurt him. He questions what she did. She says they did it together. She says she’ll tell him all about the surgeries he keeps remembering if he just opens the cell. He does. She moves closer to him. He wraps his hands around her neck. She begins speaking in Klingon and he responds in Klingon. They say a prayer to Kahless together. She wants him to say his other name. He doesn’t understand. She realizes something is wrong. He shoves her back in her cell.
Burnham finds Tyler in the mess hall. She asks him about what happened in the worker bee. He tells her about the flashbacks. She tells him to tell the captain. He refuses, saying he’d be quarantined until they got back to their own universe and Federation space. He asks her to trust him. She agrees. She is then called to engineering. Tyler momentarily blacks out and comes back to find he’s crushed his glass in his hands.
Tilly and Burnham try to piece together what they know. She downloads the information from the Klingon ship. They are in a universe where Starfleet is replaced by the Terran Empire, a Xenophobic, all-human empire ruled by a faceless emperor. The Klingons, Vulcans, Andorians, and all other life are apparently allied in a rebellion against the Terran Empire.
The Cooper approaches. Saru deduces that this Discovery has swapped places with the mirror universe Discovery. They prepare to make contact with the Cooper, but Saru informs Lorca that he is not the captain of the mirror universe Discovery. Tilly is. She takes the call as long as she can before handing it over to her “chief engineer.” Lorca takes over, putting on a Scottish accent, and covers them. They make it through that call, but realize they must prepare for the next contact.
The crew goes about studying every detail of the Terran Empire in order to use their replicators to transform the ship and themselves to fit into this universe as best as they can.
Burnham reports to Lorca about what their Terran Empire counterparts are like. Apparently, Burnham is presumed dead. Lorca killed her. Both were highly decorated. Burnham was captain of the Shenzhou. Lorca captained the Buran. He attempted a coup. Burnham was sent to stop him. Lorca killed her and the emperor destroyed his ship in retaliation. The rest of the data and information has gone missing.
Lorca has a plan to get information on the Defiant, another ship from their universe that went missing in the mirror universe. Burnham will pose as her mirror universe self, claiming to have used her death as a cover. Lorca will be her prisoner, also having survived their encounter. They’ll use this cover to get the information on the Defiant.
Tyler asks Culber to examine him to find out if the Klingons did anything to him. Tyler dodges questions about his symptoms. Stamets begins screaming to “stay out of the palace. Culber comforts him and kisses him. Stamets is momentarily lucid and warns Culber to be careful because “the enemy is here.”
Burnham helps prepare Tilly to be Captain Tilly instead of Cadet Tilly. Burnham tells her that Terran strength is a facade born of pure necessity because they live in constant fear. Tilly can depend on her crew.
The Discovery sends a transmission to the Shenzhou, now commanded by Captain Conner, and ops officer who died at the Battle at the Binary Stars in the prime timeline. Tilly tells Conner that they found Burnham and Lorca in a shuttle. Burnham says she wants her ship back. It takes some intimidation by Tilly, but Conner agrees to bring the Shenzhou to Discovery.
Culber calls Tyler to sickbay. Culber founds masses of scar tissue around all of Tyler’s tissues. They believed it to be simple torture scars. Upon further investigation. He believes its something else. He also believes there may be a second personality in Tyler. He believes the Klingons transformed Tyler physically and mentally. Tyler hears Klingon words in his head and then snaps Culber’s neck.
The Shenzhou warps to Discovery’s location. Lorca, Burnham, and Tyler are to beam over. Lorca reminds them to act Terran and they beam over to the Shenzhou.
Conner and his men meet them there. They salute each other. Conner tries to take Lorca. Burnham insists she sees to him personally. Lorca is brought to the agony booths. Burnham warns that Lorca is not to be killed.
In the turbolift, Conner tells Burnham that he almost didn’t make captain but the emperor saw something in him. He says the crew didn’t respect him the way they respect her. He needs a way to make them fear him. He attacks Burnham. They fight. Burnham comes out on top, stabbing Conner in the stomach. The bridge crew applauds Burnham when Conner’s body falls out of the turbolift. He orders Conner’s body disposed of and takes the captain’s chair. The Shenzhou warps out.
Burnham goes to her quarters and Tyler is there waiting for her. She hasn’t had a chance to access the files they need with the entire crew constantly trying to curry favor. Tyler swears that no matter what, no matter how they change, he will protect her.
Lorca is being tortured in the agony booth.