After the last spore drive jump that Stamets performed, the Discovery is trapped in a parallel universe where Vulcans, Andorians and Klingons are allies in the fight against the xenophobic Terran Empire. Stamets alternatingly enters a catatonic and a frantic state. As Lorca doubts that Dr. Culber keeps a sufficient professional distance, he decides to assign another doctor to the patient. When Tyler is sent to retrieve a computer core from the remains of a Klingon ship, his PTSD resurfaces, and he barely manages to perform the task. He visits L'Rell in the brig, who insinuates that he is not the person he believes to be. Tyler seeks Dr. Culber's help, who agrees to perform further tests on him. After the investigation of the computer data, it turns out that the ship has switched places with the Discovery from the parallel universe, here commanded by Captain Tilly. The Lorca of the parallel universe attempted to overthrow the emperor, who sent Captain Burnham of the ISS Shenzhou to apprehend the traitor. But she was killed by his followers. The emperor had Captain Lorca's ship, the ISS Buran, destroyed, while Lorca himself managed to escape. Our Lorca finds out that the USS Defiant crossed over to the Mirror Universe without the use of the spore drive, but information on the classified incident would only be accessible to active captains. He and Burnham come up with the plan to pretend that she is still alive in the parallel universe, and to return to the ISS Shenzhou with Lorca as her bounty. "Captain" Tilly calls the Shenzhou, now commanded by Connor. He reluctantly agrees to let Burnham return to her rightful place on the ship, with her valuable prisoner. While they prepare to beam over, Dr. Culber finds out that Tyler was surgically and mentally altered by the Klingons. Tyler kills him. He then rushes to the transporter platform and beams over to the Shenzhou with Burnham and Lorca. Aboard the Shenzhou, Connor tries to assassinate Burnham, but she defeats and kills him, which earns her the appreciation of "her" crew. Tyler is already waiting in her ready room, apparently ready to retrieve the information on the Defiant, while Lorca suffers in a agony booth.

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