Inside the holodeck aboard the derelict KSF Khi’eth, Dr. Hugh Culber, transformed by the program to look like a Bajoran, watches a holographic gormagander pup float through the air as a narrator explains that it holds the record of longest time spent on the Federation‘s endangered species list. The gormagander dissolves as it flies, a sign that the program is breaking down. Nearby, Su’Kal notices radiation burns appearing on the face of Captain Saru, who has been altered by the program to appear Human. Su’Kal gives Saru a piece of kelp to help with the burns. Culber reports that the program is degrading and more radiation is getting through; Su’Kal’s prior outburst caused a hull breach. Saru is wary of pushing Su’Kal too hard to turn off the program since another outburst could destroy what remains of the Federation. Culber suggests Saru connect with Su’Kal by explaining that he is Kelpien as well, despite his present appearance.
Adira Tal arrives, having beamed down from Cleveland Booker‘s ship when he rescued Commander Michael Burnham. Adira has been altered by the program to appear Xahean. They greet Saru and Culber excitedly and explain they brought additional medicine by hiding it in their mouth so the program would not obfuscate it upon arrival. Gray Tal then appears, shocking Culber and Saru, who can see him, although his Trill appearance has been altered to make him look Vulcan. The program somehow is able to manifest Gray’s appearance to others. Culber hugs him, causing a flood of emotions to pour over Gray’s face. The environment around them glitches, and Saru directs the other three to continue searching for a way out. Though the medicine brought them some time, they still only have hours left. They walk away as the Monster watches them.
At Federation headquarters, the Emerald Chain flagship Viridian fires on the complex’s shields. Inside, various Starfleet vessels trade fire with USS Discovery, which is under the control of Osyraa. Admiral Charles Vance directs the USS Voyager-J to lead the fight against the Viridian while the rest of the ships continue firing on Discovery. Kovich complains that destroying Discovery endangers the message of hope the spore drive represents to worlds that have left the Federation. Vance explains that if Osyraa uses the drive to reach those planets first, the Federation will fall.
Aboard Discovery, Osyraa orders her regulators to fire on the complex’s primary shield emitter, which is disguised as a secondary deflector array. Lt. Audrey Willa tells Vance the emitter will last less than five minutes against the barrage and Vance orders the fleet to hit Discovery harder. As Zareh brings a captured Burnham to the bridge, Osyraa tells Booker she will use a truth serum devised by Invigilator Aurellio to obtain from him the location of Theta Zeta, the dilithium planet in the Verubin Nebula. Osyraa tells the helm to prepare a course to Base 755 and for reinforcements to meet them there. Angry that her proposed alliance was refused after Vance insisted she stand trial for various crimes, Osyraa slaps Burnham and tells her they will find a way to operate the spore drive even though Burnham evacuated Stamets off the ship. A regulator reports that *Discovery’*s crew has engaged in a firefight on Deck 7. Osyraa orders Booker taken to sickbay and all regulators to evacuate to Deck 5 or above immediately.
On Deck 7, Tilly, Rhys, Owosekun, Detmer, Bryce, Lt. Ina and DOT-23 drones controlled by the Sphere data trade fire with the regulators, who retreat to the upper decks per Osyraa’s instructions. The computer reports life support is being shut off and atmosphere drained.
Lt. Teemo reports to Vance about the situation, but they are unable to detect whether there are any lifesigns in that part of Discovery. Stamets arrives and begs Vance to stop firing on the ship as it is the only way to return to the dilithium planet and save the away team in time. However, Vance rejects his request as he cannot fall under Osyraa’s control and orders him loaded aboard the Foresight, an emergency transport ferrying civilian away from the battle. The complex’s shields start to fail, allowing the Viridian to provide cover fire for Discovery to escape. A surprised Teemo reports that a Ni’Var fleet is about to arrive to provide backup. The Ni’Var Razors are blocking *Discovery’*s path out, so Osyraa directs the Viridian to deploy its cache of pesticides, targeting shuttlebays, airlocks and other intakes. Burnham asks her not to, saying she will convince Vance to stand down. Vance is skeptical but after Burnham’s urging relents. Burnham is escorted off the bridge as Discovery and the Viridian go to maximum warp, pursued by Starfleet and Ni’Var vessels.
Saru, increasingly suffering from the radiation, offers to prepare a meal with Su’Kal, specifically sài-núù-lem, a traditional Kaminar dish. Su’Kal says he has always wanted to make it but that the holograms did not provide good instructions. Saru offers to teach him, revealing he learned the recipe from his father. Su’Kal questions how a Human holo could eat sài-núù-lem and Saru explains that he is not as he appears and is in fact Kelpien, though Su’Kal seems doubtful.
In the Discovery sickbay, Aurellio explains the serum will take time to work on Booker, but Zareh argues that he has built up an immunity and demands to use the neural lock device instead. Osyraa directs Aurellio to give Zareh the neural lock, but the scientist protests, arguing that Booker’s empathic abilities mean the device will cause him excruciating pain. When Aurellio resists her, Osyraa chokes him until he loses consciousness. Zareh places the neural lock on Booker as Osyraa tells Burnham that the sheer agony will eventually kill him. Zareh tells Burnham she is trapped in a no-win situation, but she says she does not believe in those as Booker screams in agony.
As they eat sài-núù-lem around a fire, Saru explains to Su’Kal that the holoprogram has changed his appearance and changes other things as well. Su’Kal notes that a nearby door previously only led to Kaminar, but that recently it has led to several other worlds. Saru asks why Su’Kal is uncomfortable discussing the “outside” world. Su’Kal explains that he was to be rescued by people from the outside and asks why, if it exists, help never came. Saru explains that the Burn has prevented long-distance travel for many years and that his mother, Issa, created the holoprogram to keep Su’Kal safe, though she never intended him to live there for his whole life. Su’Kal stands and walks through the door to the holo Kaminar.
Osyraa and Zareh torture Booker as Burnham pleads with them to stop. They pause to let him recover, and Osyraa explains that Tilly and the rest, facing oxygen deprivation, will be unconscious in 30 minutes and dead within an hour. Osyraa tells Burnham she cannot save them, but can save Booker at least. She resumes torturing Booker, and Burnham gives in, asking Osyraa to stop. She approaches Booker’s bedside, ostensibly to apologize for causing him pain, but suddenly taps a command console, enshrouding them inside a quarantine forcefield. With Osyraa and Zareh cut off, Burnham and Booker escape through a side door.
In engineering, Tilly and the other officers are having difficulty breathing and are sharing among them one oxygen mask. Owosekun explains that she has superior lung capacity having grown up free-diving for abalone in the Ogbunike Caves. Bryce reports he cannot bypass the Emerald Chain’s command functions. A hard reboot could restore computer systems, but they would have to access the data core. Burnham and Booker, grounded by the transporter jammer, sneak through the ship, avoiding regulators. Burnham has a plan, but tells Booker it will “make Sigma X feel like a vacation.” Using one of the regulator’s badges, Burnham broadcasts an audio message across the ship: “It wasn’t a particularly happy birthday. But I found you. We played seven-card stud and drank synthehol. Too bad we didn’t have fireworks. Nothing stops a party like fireworks.” Tilly explains to the others that on her birthday the year prior, she was going “through a hard time” and hid in a nacelle after her shift. Burnham found and distracted her until midnight, never mentioning Tilly’s birthday. Tilly realizes Burnham’s message was directing her to use an incendiary device to destabilize the superconductors holding the now-detached nacelle in place, knocking the ship out of warp and allowing the pursuing fleets to catch up. Rhys notes the DOT drones would not be able to set the bombs due to the magnetic fields, meaning the officers must undertake a potential suicide mission themselves.
Burnham and Booker, meanwhile, make their way to the data core.
Su’Kal returns and reports that the holoprogram’s Kaminar is empty. Saru explains that the outside world is composed of many worlds and non-holographic individuals, including himself. He further explains that he once had an opportunity to leave the only home he had ever known, and was similarly afraid. The world can be challenging, Saru admits, but is also “beautiful and diverse and filled with wonder.” Issa’s program was meant to help him understand it. Suddenly, the Monster arrives. Saru explains that it is actually trying to help him confront his greatest fear —whatever is behind the locked door— or they will all die. Su’Kal flees, searching for the elder Kelpien holo.
Culber, Adira and Gray search the rest of the program but find no one. They come across a section where the hologram is breaking down into the real world, and Gray offers to explore it since he cannot be harmed by the radiation. As he does, Culber explains to Adira that he believes Su’Kal is a polyploid, which is caused when normal chromosomal separation is disrupted in utero. It causes the individual’s genes to be affected by their environment, which for Su’Kal included huge concentrations of dilithium and radiation. Culber explains that Su’Kal’s genes mutated to interact with dilithium in unique ways. Adira notes that dilithium has a subspace component and speculates Su’Kal has a genetic connection to subspace as well. Culber realizes Su’Kal caused the Burn and nearly caused another earlier when he was upset by the Monster.
Adira postulates that his scream traveled at the resonant frequency of dilithium’s subspace components. Culber notes that whatever frightened him 125 years ago must have been much worse than his recent experience. However, if they remove Su’Kal from the dilithium planet, Culber says another Burn can be prevented. Gray returns and reports that the Khi’eth is falling apart and its shields will not hold much longer.
Su’Kal discovers the elder holo is gone, which Saru explains is a sign of the program’s severe degradation. The others arrive, and Saru prompts Su’Kal to take them into the locked room.
Aboard Discovery, Tilly and the rest struggle against hypoxia to reach the nacelle access point. All but Owosekun have collapsed, unable to move. Tilly orders her to take the remaining oxygen and deliver the bomb by herself. Taking the bomb, she tells her fellow officers she loves them all and ascends toward the nacelle access.
Elsewhere, Booker trades fire with regulators as Burnham tries to gain entry to a turbolift. They make it inside, but are unable to depart. Moments later the regulators open the door to find the cab empty; Burnham and Booker opened the other door, exposing the inner belly of Discovery, as they stand on the outside of the cab’s platform.
Burnham shoots one regulator through the ceiling panel and in the ensuing firefight falls into the cab. Knowing time is short, Burnham jumps out and falls to another turbolift cab as it travels through the open mechanical areas of Discovery. Booker remains behind, fighting the regulators and Zareh as his own cab moves into the underbelly with its doors open. Burnham forces open the door of the cab she’s riding and takes the single regulator inside by surprise and rerouting to the data core. She arrives at the data core and engages in combat with Osyraa and several regulators.
Owosekun arrives at the nacelle access point having run out of supplemental oxygen and barely able to stand. She places the bomb but, weak, collapses before leaving. One of the DOT-23 drones controlled by the Sphere data arrives, urging her to stand. It reaches out for her just as the bomb explodes. The nacelle now destabilized, Discovery is forced out of warp.
Discovery reenters normal space, careening wildly off its axis. The turbulence knocks Burnham and Booker, as well as their respective assailants, to the ground. The Viridian surrounds Discovery inside a large internal chamber to protect from the pursuing force. As their turbolift flies through the ship’s underbelly, Zareh taunts the prone Booker, asking whether he or “that fat cat of yours” would leave a bigger stain from a fall. Booker, however, manages to eject Zareh from the cab instead, and he falls to his death. “She’s… a queen!” Booker shouts after him.
In the data core, Osyraa manages to force Burnham into a wall made of programmable matter. Burnham says they do not have to be enemies, but Osyraa responds that trying to negotiate with Vance was a mistake. She pushes Burnham backwards into the wall, which subsumes her. Osyraa looks on satisfied, but a few moments later Burnham shoots her dead from inside the wall before emerging and coughing up some of the matter. “Yeah, well, unlike you, I never quit,” she says to the dead Osyraa. Burnham performs a hard restart of the computer systems, restoring the Starfleet programs. Having regained control, she restores life support on all decks, beams the regulators off Discovery and diverts auxiliary power to shields. On a shipwide channel, she orders any crew who can hear her to report to the bridge.
Her message awakens Owosekun, who is surprised to find herself safely back with her comrades. She realizes that the badly damaged DOT-23 drone next to her saved her life. The rest awaken and celebrate before departing for the bridge.
There they discover Booker, Aurellio and Burnham, who informs them they are back to using the same operating system from before Starfleet’s futuristic upgrades. The Viridian begins firing anti-proton cannons at Discovery. Burnham fills Tilly as acting captain in on their situation and says she has an idea, but Tilly tells her they need her leadership now, surprising Burnham. Burnham explains her plan: To eject the ship’s warp core to blow up the Viridian while using the spore drive to jump back to the nebula and save the away team.
Aurellio explains to the confused officers that while Stamets needed tardigrade DNA to operate the drive, Booker’s Kwejian empathic abilities allow him uniquely to connect to other species and operate the spore drive. The crew ready, Burnham has Bryce warn the allied fleet to back off of the Viridian and sends Book to the spore cube.
The Monster leads Su’Kal, Saru and the rest toward the locked room, though Su’Kal says he is afraid to turn off the holo. Culber tells him it is okay to be afraid. They enter, but Gray becomes upset after realizing that he will disappear when the holo is deactivated. Adira will still be able to see him, but he realizes he needs a physical connection to the world. Culber promises to find a way to let him be seen by everyone.
Su’Kal turns off the holo for the first time in 125 years. As the simulation deconstructs, Adira and Culber watch sadly as Gray disappears.
They are left looking at the degraded ship and inhospitable planet. Adira and Culber have been returned to their Human appearances. Su’Kal asks the computer to show him what he saw that upset him so greatly. The computer displays a holo-recording of Issa, near death, and a young Su’Kal. The young Su’Kal asks her what happened to their family, lying nearby under sheets, and she explains they did not survive. With her last breaths, she warns him not to touch the console until Federation rescuers arrive. She then falls over, dead. The sight upsets Su’Kal so much that he cries out in agony, inadvertently causing the Burn.
The older Su’Kal is left despondent. Behind him, Saru tells Su’Kal that he is no longer alone. Su’Kal turns and for the first time sees Saru’s true Kelpien appearance, gasping in surprise. Culber calls for Discovery but gets no response.
On Discovery, Booker promises to tell Burnham the story of his “mentor,” from whom he derived his new name, if they survive, explaining that he tries to live up to that name every day. Discovery ejects its warp core and engages the spore drive to jump away. The explosion blows apart the massive Viridian.
A holo-recording made by Issa thanks the rescuers for finding Su’Kal and asks them to return him to family on Kaminar in the Narrows Firth. Su’Kal asks how he can avoid causing another burn; Culber explains it won’t happen once he leaves the dilithium planet. Saru explains the Burn was not his fault but that he will help him understand. The ship shakes and warns of imminent structural failure. The four embrace, but are saved when Discovery suddenly arrives. Moments later, the Khi’eth completely falls apart.
Discovery returns to Federation headquarters as Su’Kal looks on in amazement. Burnham arrives in sickbay to witness Culber and Adira greeting Stamets. Though she smiles at him happily, Stamets turns from her icily. In engineering, Owosekun and Reno repair the damaged DOT-23 drone.
Without Osyraa, the Emerald Chain quickly fractured.
*Discovery’*s success has also prompted renewed interest from non-Federation worlds, including Trill, which quickly rejoined, and Ni’Var, which was considering membership. Saru returns to Kaminar with Su’Kal “to build a new life,” according to Burnham. Outside Vance’s office, Burnham happily runs into Aditya Sahil, who has been made a lieutenant. In his office, Vance notes that he once berated Burnham for doing things her own way, but notes he will “be damned if your way didn’t work, too.” Burnham challenged him to think of fresh ways to act in the 32nd century, and he asks her to captain Discovery, surprising her. Burnham asks for him to wait to make such a decision until Saru, who supports her captaincy, returns from Kaminar, but Vance says Discovery has a vital mission to spread fuel from the dilithium planet to wanting worlds that cannot wait.
Burnham, now donning a captain’s rank and new Starfleet uniform, steps onto *Discovery’*s bridge where the crew greets her. Most look happy, including Adira, who now wears a Starfleet science uniform, but Stamets maintains a more neutral facade. As Burnham takes the captain’s chair, Tilly reports that the mining vessel Coloma awaits them at the dilithium planet, after which they will deliver dilithium to two Federation worlds, five non-Federation worlds and a starbase that Tilly notes wants to thank them “with their signature gelato.”
“Let’s fly,” Burnham says. Discovery exits the distortion field and warps away.