During a briefing at Federation Headquarters, Captain Saru reports to Admiral Charles Vance and other Starfleet officers about the progress of repairing USS Discovery and retrofit with technologies from the current time, including upgrades to internal systems, the addition of programmable matter and the detachment of the ship’s warp nacelles, which will improve manoeuvrability and efficiency.

Vance welcomes Discovery back to the fleet and assigns several other matters to other captains at the briefing. Saru notes that Discovery could carry out these missions more quickly, but Vance says that he needs the ship to remain at headquarters as a “rapid responder,” revealing to the other captains the still-classified existence of the spore drive. Vance also reports that the planet Argeth is on yellow alert due to a potential impending threat from the Emerald Chain in the forthcoming 12 hours. Osyraa and the Chain “have been pushing hard lately,” Vance says, but their objective on Argeth remains unclear. Vance wants Discovery ready to jump in if needed.

On Discovery, the bridge crew puts on their new combadges, which Lt. Audrey Willa explains are also holo-PADDs, tricorders and personal transporters. She also says their stations have been upgraded with programmable matter. Suddenly, Linus beams into the captain’s chair, having intended to beam himself to the mess hall. Bryce reports a hail from a vessel outside the Federation’s distortion field. Nilsson orders him to answer, and the crew is surprised to find Grudge alone aboard Cleveland Booker’s vessel.

After pulling the ship in, Saru and Michael Burnham listen to a holographic recording left by Book. He explains that he had been travelling to the Bajoran exchange when he learned about the existence of a black box like the ones Burnham had sought in her investigation into the Burn. The box is on Hunhau, a planet controlled by the Emerald Chain. Booker said he planned to investigate, and would program his ship to deliver Grudge to Discovery if he did not return within 24 hours. Ending the recording, Book says he will find Burnham and Grudge.

Burnham tells Saru the message was recorded three weeks prior, indicating something had gone wrong. She also explains that she previously had found two black boxes that showed their ships did not explode at the same microsecond in the Burn. If the third recorder found by Book also showed variance, it would indicate the Burn had a point of origin that could be triangulated. Saru tells Burnham that Discovery cannot leave Federation headquarters due to the potential crisis on Argeth. Burnham retorts that the Federation will not stabilize until questions about the Burn are answered, and also, Book’s life is in danger. Saru tells her the Federation has become vulnerable and that Vance would not approve such a mission based off of “a cat in a ship.

Later, in a corridor, Burnham tells Philippa Georgiou that she cannot stand by and is planning an “unsanctioned mission” to find Book and the black box. Georgiou notes that such an action would reflect poorly on Saru, who is trying to prove his legitimacy to Starfleet command. “I’d rather regret something I did than something I didn’t,” Burnham responds. Georgiou says Burnham had hooked her when she said “unsanctioned mission.

Act One

Burnham and Georgiou take Book’s ship to Hunhau, a sparsely populated M-class planet used for ship salvage. Georgiou asks why Burnham fell in love with Book. Burnham is surprised by the question, but Georgiou says that given how Burnham’s relationship with Ash Tyler turned out she understands why Burnham has resisted her feelings for Book. Burnham protests that she had her own ship and “didn’t love here or anything,” having inteded to say “live here.” Georgiou latches onto the slip, but Burnham protests and says Georgiou’s stunted emotional range does not qualify her to judge Burnham.

Georgiou suddenly has visions from the Terran universe of herself over a bloody body saying the name “San” despondently. Burnham notices Georgiou had frozen again, as she had in the corridor following the mission to the USS Tikhov, but Georgiou dismisses her concerns.

Just then, they arrive at Hunhau; Burnham explains that ship graveyards such as the one in orbit of Hunhau surround many planets. The populated area is heavily shielded, obscuring lifesigns. They are hailed by an Orion, Tolor, who asks why they have come. Georgiou responds sourly that they have come to purchase salvage and pressures him into authorizing their landing by telling him she has dilithium to barter. Burnham explains to Georgiou that she has reverse-engineered Book’s system to track Grudge’s cat collar to seek out his receiver.

On Discovery, Tilly is surprised to find Grudge in the quarters she shares with Burnham. She learns from the computer that Burnham is no longer on board and that Book’s ship is gone as well.

Burnham and Georgiou land on Hunhau, where salvaged ships and platforms float in the air. Tolor confiscates their weapons and says they will not receive their merchandise until they hand over the dilithium. He explains that Osyraa, the facility’s owner, is his aunt and that the workers owe Osyraa and are effectively slaves.

Inside the facility, an Andorian named Ryn implants a device into the back of a worker’s neck. He apologizes but the worker calls him a traitor and leaves. Nearby, Book works with an injured Bajoran named Lai who also dismisses Ryn. Book tells him the workers know Osyraa would kill him “real slow” if he does not cooperate. Book then spots Burnham and Georgiou.

Lai steals a water ration, provoking Tolor and the guards. As Burnham and Georgiou watch and Book is held in protest, Tolor shoots his weapon at Lai, forcing him to run toward the facility’s fenceline perimeter. When he passes it, the implant in the back of his neck explodes, decapitating him.

Act Two

On Discovery, Adira Tal works in engineering as her vision of Gray Tal protests that she only ever works, eats and sleeps. Lt. Cmdr. Paul Stamets enters, complaining that someone moved the intermix pods in a way that does not make ergonomic sense. He protests that engineering is “in chaos,” but Adira explains that she redesigned the spore drive interface so Stamets would no longer need to use the shunts on his forearms. Instead, the drive can now be controlled via nanogel, which acts as a quantum transducer. Stamets is grateful.

On Hunhau, Georgiou praises the efficiency of the fencing system and asks to purchase it, but Tolor tells her it is a proprietary technology owned by Osyraa. Burnham quietly asks Georgiou to buy her some time; Georgiou distracts Tolor with additional questions while Burnham approaches Book, who takes her to a secluded storage area. They embrace, and he explains he also has the implant and that he still has the black box hidden in the wall of his quarters. However, Burnham says she will not leave without Book. He warns that Osyraa is ruthless, explaining that Ryn was born into the Emerald Chain but that after fomenting dissent she cut off his antennae and forced him to install the explosive implants, making him a pariah among the workers.

Tolor and Georgiou find them, and Burnham explains covertly that she will need 40 minutes to clean and inspect the parts, the time left until the shift change. Tolor is distracted by a nearby accident, and orders a drone to follow Georgiou and Burnham around instead.

On Discovery, Tilly directs several crew members in engineering, where she is ensuring the spore drive is fully operational in case the ship needs to jump. Saru pulls her aside and asks about Burnham’s disappearance, but Tilly says she must have had a reason to leave them in the dark. Saru reveals she violated his direct orders by leaving, alarming Tilly.

Saru confides that he has not felt as mistrustful of Burnham since they served on the USS Shenzhou. Tilly suggests Saru inform Vance of Burnham’s disobedience, saying she does not want the entire crew to be thought less of if Vance does not learn of it from Saru. Tilly says she may also have acted as Burnham did in that situation, though Saru says he does not believe that.

On Hunhau, Burnham uses a piece of metal to bash the drone apart, almost immediately setting off alarms. Georgiou uses the drone’s parts to build a weapon. Meanwhile, Ryn has informed the other slaves of the plan, though he says he does not know how many believed him. Ryn also retrieved the black box, a small cylindrical device, from Book’s quarters.

Act Three

At headquarters, Saru informs Vance and Willa of Burnham’s unauthorized mission and assures him they remain ready to respond if needed to Argeth. Vance tells Saru that talks with the Emerald Chain have broken down and that Discovery may need to jump shortly, even if that means without the ship’s first officer.

On Hunhau, Burnham and Georgiou have allowed themselves to be taken captive and brought by Tolor aboard Book’s ship. A henchman has found the 30 units of dilithium aboard. Just then, the shift change in the salvage facility prompts Book, Ryn and the other workers to riot and escape, though they are pinned down and cannot pass the fence until it is deactivated. Burnham and Georgiou fight Tolor and his men, but just as Georgiou is about to use her constructed weapon against Tolor, she is incapacitated by another flashback and falls to the floor.

After Burnham struggles with Tolor, Georgious comes to and retrieves the fence controller before he beams away. She deactivates the fence, allowing the slaves to escape to a waiting transport vessel. However, Ryn is injured by an energy beam and Book remains behind to help. Burnham and Georgiou use Book’s ship’s weapons to fight off the guards and beam Book and the injured Ryn aboard.

Georgiou shoots down several of the floating vessels, heavily damaging the facility, before they and the transport vessel warp away to safety. Book gives Burnham the black box, which he had hidden away inside his glove.

Act Four

Burnham tells Georgiou they need to talk about what his happening to her. Burnham is alarmed to find Georgiou does not know, though she admits it has been happening for weeks and is becoming more severe. She tells Burnham not to tell anyone else, but Burnham responds that she is not in the Terran universe anymore and “vulnerability is not a death sentence.” Georgiou says she remembers another Michael Burnham telling her trust her, “and we know how that ended.”

In Discovery‘s mess hall, Stamets sees Adira seemingly talking to herself and says he is curious. Adira tells him she can see Gray. Stamets says she is holding onto someone she loves, as he did after Hugh Culber was murdered and before he was resurrected from the mycelial network. The experience changed his mind about life and death as a linear phenomenon, and that he believes Adira can see Gray. Adira tells Stamets she can do something about his shunt.

In Sickbay, Book tells Burnham that Dr. Tracy Pollard said Ryn will recover and that his neck device has been removed. He notes that it is time for Burnham to “face the firing squad” as the two enter a turbolift. He says he hopes she doesn’t regret having put her in this situation and stops the turbolift. He thanks her for saving his life, and the two nearly kiss but are interrupted by Linus, accidentally beaming into to turbolift, having meant to go to the science lab. He leaves and Book passionately kisses Burnham.

In their quarters, Stamets and Culber celebrate the removal of his shunts. Stamets explains that he reached out to her because of her intelligence and isolation, but also because he could connect with someone whose lover had died, something he never expected he could find.

At headquarters, Vance chastises Burnham’s rogue mission, but also tells Saru he may have approved the mission given the unique opportunity to gather intelligence. Vance tells Burnham that she does not get a pass for having saved the future, and that she would be in the brig, but that she saved lives on Hunhau.

Burnham tells Vance that the Federation cannot heal until the cause of the Burn is discovered. Vance leaves Burnham’s punishment to Saru, who sorrowfully relieves her of her duties as first officer, leaving her as chief science officer only. She tells him he is doing the right thing. He leaves, and a tearful Burnham removes her combadge and holds it in her hand.