Lorca speaks to his followers after freeing them from their agonizer booths. One is Mirror Commander Landry. He promises to reclaim their Empire. Landry suggests retreating and regrouping. Lorca assures her that he a plan.
They head to Stamets’ laboratory and find Mirror Stamets hiding there. It was Stamets who sold Lorca out to the Emperor. When the Emperor’s ships caught up to his, he fled into an ion storm. The interference caused a transporter malfunction that sent Lorca to the prime universe. Lorca tells Stamets to show him the biological weapon he has been developing for the Emperor. Lorca begins using it on the Emperor’s soldiers onboard the palace ship.
Emperor Georgiou orders Burnham taken to the brig. She manages to escape the throne room.
The USS Discovery is still en route to the ISS Charon. Stamets was unable to save the ship’s mycelium crop. He’s worried that Mirror Stamets already infected and destroyed their crop beyond repair. Stamets explains that the Terran Empire is using the network to power the ISS Charon and give it unimaginable power, but the way they are extracting the power is poisoning the network.
Lorca gets on Charon’s shipwide speaker and invites anyone onboard to join his renewed coup against the Emperor and make the Empire glorious again. He orders that Michael Burnham isn't to be touched because she’s integral to his plans. The Emperor gets a lock on Lorca’s location and marches towards him with her men.
Georgiou’s group meets Lorca’s group in a hallway. They open fire on each other. Lorca’s group wins the firefight, but Georgiou makes an emergency transport and gets away. Stamets says he can disable her ability to do that again.
Bur is in a Jeffries tube trying to make contact with the Discovery. She’s successful and gets Saru on video. She tells Saru about Lorca. Stamets realizes that Lorca altered the coordinates of his final jump and brought them all to the mirror universe. She tells them that Lorca is staging a coup and that they should all stay away from the palace ship. She’s worried that the Terran Empire may try to invade their own universe, but Stamets says that won’t matter if the mycelial infection spreads and destroys all life. Stamets believes firing a torpedo on their ship’s engine would sever the ship’s ties to the network. There’s a containment field around it. She tells them she’ll get it down and signal them when its ready.
Lorca’s men enter the Emperor’s throne room and take’s the Emperor’s sword. He has Mirror Stamets killed. Landry received word of Burnham’s unauthorized transmission to the Discovery.
Burnham is making her way through the Charon’s decks when Lorca makes contact with her. He invites her to stay with him and abandon the failed, idealistic social experiment that is the Federation. He wants her to stay with him and bring peace to the mirror universe. He tells her that her gifts outstrip even those of the Mirror Burnham. By the time Landry closes on her position it turns out that Burnham has already rerouted the feed and moved to another location.
Burnham finds Georgiou in the Emperor’s sanctuary. Burnham tells Georgiou that she wants nothing to do with Lorca’s idea of destiny, that she will forge her own past. They compare their mirrored loses. All versions of Burnham seem to have betrayed all versions of Georgiou. Burnham vows to stop Lorca. Georgiou asks what her plan is. She wants to disable the orb’s containment field. Georgiou notes that those controls are in the throne room.
Stamets tells the Discovery crew that the run on the Charon’s engine is more complicated than they thought. They’ll have to use all of the energy from their remaining spores to puncture it, which means they can’t get home. They’re unlikely to be able to dodge return fire at that distance. The resulting blast would also incinerate them through the Discovery’s shields. Saru gives an inspirational speech about their capabilities and not accepting a no-win scenario.
Lorca has the ship under control. Burnham and the Emperor come to the throne room. Burnham says she knows for Lorca to claim the throne he must kill the Emperor. She offers the Emperor and herself in exchange for letting the Discovery leave safely.
Tilly thinks she’s found a way to solve their problem. She believes they can ride the mycelial shockwave that will result from the destruction of the Charon and let Stamets navigate them home.
The Discovery receives Burnham’s signal to drop out of warp. They come out near the Charon and get a transmission from Lorca. He tells them that his admiration for the crew was always sincere. Burnham confirms that she will be staying behind. She says “this is my place.” Saru realizes that she’s telling him where to aim and opens fire on the Charon. Brunham and the Emperor make their move and the battle begins.
The fight ends with Burnham holding Lorca at gunpoint. She tells Lorca that they would have helped him get home if he had just asked. That’s who Starfleet is, who she is, and why she won’t kill him now. The Emperor doesn’t have such reservations and stabs Lorca in the chest. She kicks his body into the heart of the mycelial engine.
Lorca’s troops try to storm the throne room. Georgiou knows her reign is over and offers to hold them off while Burnham completes her mission. The Discovery beams Burnham back aboard, but she grabs Georgiou and takes her with her.
Discovery goes on black alert. Landry orders the Charon to target the Discovery. The Discovery makes its run and fires on the mycelial engine of the Charon. They go to warp to try to ride the wave. The spore drive fires up and they make the jump into the network. Stamets sees the network regenerating. Stamets becomes lost in the network. He remembers what Culber told him in the network, to follow the music and open his eyes. He sees the “clearing in the forest” and gets them back on track.
The Discovery comes out of the network. Stamets confirms they are in their own Alpha Quadrant. However, they overshot the date by nine months. They get no response from the Federation when they try to make contact. They bring up a map of the Alpha Quadrant and discover the Klingons have won the war.