A Discovery technician finds Stamets cradling a dead Culber in a corridor she is repairing.
Burnham is getting stressed with the Mirror-Universe charade. Mirror-Saru is her enslaved personal spa attendant. The Terrans execute crew members for thought crimes by transporting them into space. She confides her concerns to Tyler in bed and Tyler calls her his tether just as Mirror-Saru enters.
Tilly contacts her from the Discovery and turns the transmission to Saru so they can discuss how Burnham can get the Defiant intel to the Discovery. Saru asks if she has seen any Kelpians and Burnham lies rather than tell him that his Mirror-self is a slave.
On board the Discovery, Saru and Tilly discuss whether they should tell Burnham about Culber's death. Saru decides against it as it would distract her from her mission. The suspicion is that Stamets killed Culber and he is being kept in restraints. Tilly wants to treat Stamets using her understanding of the spores.
On board the Shenzhou, Burnham is given the location of the leaders of the Rebellion including the Klingon leader called the Fire Wolf and is ordered to destroy the planet Harlak and all the rebels. First Officer Detmer begins giving the order to prep torpedoes and Burnham reprimands her, cancels the order, and explains that she'll be getting intel before they destroy the base.
In the brig, Lorca advises she give the order to destroy the planet in order to further their mission. Burnham feels that the torture he is undergoing is affecting his judgment. She feels if she can find a way to negotiate with the Klingons, they might be able to solve the conflict in their own universe. He sees her perspective and tells her to take Tyler down to Harlak.
Tilly explains to Saru her plan to use spores to heal Stamets' brain. He gives her permission to begin treatment.
Tyler and Burnham beam down to the planet and are immediately attacked. Burnham gets Tyler to surrender to the rebels in order for them to get a meeting with the Fire Wolf. The Fire Wolf turns out to be Mirror-Voq. This causes Tyler to retreat from the group. When Burnham offers them the chance to evacuate before the Shenzhou fires on the planet, he calls on their "prophet" to read her intentions and it turns out to be Mirror-Sarek.
Mirror-Sarek mind melds with Burnham and sees her past, including her relationship with his other self. He declares her intentions to be true and peaceful. Before the evacuation begins, she asks Mirror-Voq how he was able to unite the species against the Terrans. As he explains, Tyler begins to get flashbacks again and attacks Mirror-Voq.
Voq wins the battle and moves to kill him, accusing Burnham of deception. Sarek speaks up and reasserts that Burnham has good intentions. The rebels give Burnham a data core with important intel which they will render useless by the time the coding is decrypted. Leaving the base, Burnham has the Shenzhou beam her and Tyler back. She turns the core over to Detmer and orders the torpedoes be readied. She orders Tyler to join her in her quarters.
On the Discovery, Saru and Tilly are overseeing the spore treatment which seems to be working on Stamets. Saru expresses admiration for Tilly's work just before Stamets' vitals begin to spike. Against Tilly's recommendation, he orders medical staff into the spore chamber to try to revive him. They are unable to resuscitate him.
In her quarters, Burnham demands that Tyler come clean about what is going on with him. He confesses that L'Rell has some sort of hold over him. He explains that he tried to be human for her but seeing Voq finally tipped the balance and he has accepted who he really is. He admits to killing Dr. Culber. Burnham doesn't believe him but then he reminds her that they fought each other on T'Kuvma's ship. She realizes that he really is Voq.
He remembers that she killed T'Kuvma and attempts to kill her in retribution but Mirror-Saru intercedes and throws him to the Imperial Guards who subdue him and take him to the brig. Burnham sees Detmer and confirms the order to prepare Tyler's execution by transporter.
Tilly is mourning Stamets' death when he show signs of reviving. In his mind, he meets his Mirror-self in a mycelial forest.
In the Shenzhou's transporter room, Detmer is pronouncing sentence on Tyler. Burnham offers him a chance to say something in his defense and he responds in defiant Klingon. She gut-punches him and then operates the transporter herself to execute him. She orders Lorca be brought to her ready room.
Tyler is transported into space but almost immediately re-transported by the Discovery where he is put in the brig by Saru after he removes the disc containing the U.S.S. Defiant intel that Burnham had hidden on Tyler before transporting him.
While Lorca and Burnham are discussing their next step, the bridge informs her that another ship is targeting the planet. Because the rebels haven't had time yet to complete their evacuation, Burnham tries to stop the ship but is unable to see or communicate with it. The torpedoes are launched and the planet is destroyed.
An incoming transmission from the Emperor reveals the Emperor is, in fact, Mirror-Georgiou. She reprimands Burnham for the delay in following her orders and reminds her that one must bow to one's Emperor. Burnham complies as Lorca looks on.