Russell and Simone Lightbourne summon Kaylee Lee into the palace, where they show her her family's Mind Drives that Jade recovered. However, before giving them to her, Russell once again asks Kaylee why they stole the transport ship. Kaylee says it was for shelter but Josephine Lightbourne appears and calls out Kaylee for lying. Josephine reveals that Kaylee's family was running away from Sanctum. Kaylee denies the accusations, mistaking Josephine for Clarke Griffin. Josephine then calls Kaylee by her old nickname "Lee-Lee," making Kaylee realize that it is really her childhood friend. Josie reveals that it was Kaylee who killed her, and Kaylee admits it, saying that she did it for the sake of Sanctum. To get even, Josephine takes a knife off the table and stabs Kaylee, killing her.
After killing Kaylee, Josie goes to change into her dress. Her parents find her dancing while painting. Russell asks for her to spy on the group from Earth by pretending to be Clarke in order to figure out who among them are Royal Bloods. Josie says it would be much easier to get hosts if they ran the "breeding" program she had proposed. Her parents say it wasn't ethical, but she questions whether Clarke actually consented giving up her body. Russell is worried that if the Earthlings found out the truth, they would burn Sanctum, just like they destroyed Earth. So, Josie agrees to pretend to be Clarke. For failing to save Rose, Jade is assigned to be Josie's bodyguard.
Jordan becomes suspicious that something happened to Delilah during the Naming Day because she doesn't remember him anymore or even her favorite flower. However, Bellamy and Murphy tell him that it's nothing to worry about. They are joined by Josie, pretending to be Clarke, who shocks them by suddenly allowing Madi to go to school. Speaking in Trigedasleng, Gaia questions "Clarke's" sudden change of mind, as it risks exposing Madi as a Nightblood. Josie doesn't understand Trig and therefore doesn't figure out Madi is a Nightblood. Josie also agrees with Murphy and Bellamy that Jordan should stop worrying about Delilah. Jordan asks Blythe Ann about Delilah's sudden change, but she just tells him that Priya's return is a blessing. When Bellamy asks Jordan to drop it, Jordan says that he won't give up on Delilah the way Bellamy gave up on Octavia.
Next, Josephine goes to introduce herself to Clarke's mother, Abby. She finds Abby arguing with Jackson about her obsession with saving Kane since it's impossible to perform a successful surgery before Kane dies. Abby says she needs to bring Kane back because out of all of them, he is the only good person who deserves to live. Josie joins them and begins to help Abby. Abby asks "Clarke" to take notes and is surprised to see her using her right hand since Clarke is left-handed. Abby asks if Clarke is feeling okay. To avoid blowing her cover, Josephine stops taking the notes and offers different suggestions, before leaving.
Bellamy shows Murphy a map of Sanctum and explains the plan to have Raven build their own radiation shield. The intention is for the Earth people to eventually move in there since they can't stay in Russell's settlement forever. While they're still talking, everyone is asked to gather to say goodbye to Rose as they offer her to the woods. After noticing that Jordan is missing, Bellamy and Murphy agree to ditch Rose's funeral and see what Jordan is up to. Bellamy correctly suspects that Jordan went to investigate the room where Delilah became Priya VII. Gaia joins them. When Josephine sees them heading into the room, she follows them hoping to stop them before they learn too much.
The room is filled with human skeletons. Gaia notices that the skeletons have the Flame infinity symbol and wonders if they were all Commanders. Bellamy figures out that the skeletons belong to the original Eligius III Mission Team Alpha. He notices that some of the skeletons belong to kids, and Josie explains that people will do anything to ensure survival for the ones they love. Josie starts urging them to respect Sanctum traditions and leave, but Jordan continues exploring. He finds a secret door which leads to a lab.
In the lab, they notice much technology related to Becca and the Flame. Bellamy explains that they can't be commanders because Becca didn't invent the Flame until after the first apocalypse. However, she provided her technology to Eligius Corporation before then. Jordan finds a computer and unlocks it using the Eligius III access code. They find a list of video logs. Gaia suggests playing the one titled "Eureka!" It is an experiment log:
- A doctor named Gabriel Santiago is forcibly experimenting on a young woman named Brooke, who was grown from an embryo in the incubator. He has spent the last 25 years trying to figure out how to bring Josie back to life. Russell's original form is by Gabriel's side, encouraging him. Gabriel believes he has finally figured out why he was failing before: to successfully upload Josie's consciousness to a new host, they needed to use a fully developed brain and clear the host's consciousness first in order to accommodate the full memory size. He takes out a Mind Drive, which looks a lot like the Flame. He mind wipes Brooke and inserts the Mind Drive inside her neck. A few moments later, Josie wakes up violently inside Brooke's body. Josie notices how much her boyfriend Gabriel has aged. She is afraid of her dad at first, but Gabriel explains that his dad only killed her because of the eclipse-induced psychosis that Josie had figured out. Josie is proud of Gabriel for reverse-engineering the Mind Drive technology to be able to upload an entire mind into a new host. Gabriel ends the experiment video by saying that Josephine was the first success, with more to come.
After watching the video, Bellamy, Jordan and Gaia are horrified by the inhumane experiments they just witnessed. They figure out that the Primes have been manipulating Nightbloods to sacrifice themselves to false gods. Jordan calls it murder, but Josie claims the hosts were willing, even though Brooke was forced into it. Murphy is indifferent, claiming that back on Earth, they used to arm kids to go to war. Realizing that Madi is in danger, Gaia says she will go get her back from school. This helps Josie figure out that Madi has royal blood. She offers to come along.
Later on, Josie tells her parents that Clarke's friends have figured out about the Primes and that Clarke wasn't the only Nightblood. She also mentions that the earthlings call hosts "Nightbloods," which she seems to prefer. She suggests snatching them and turning all the Nightbloods into hosts by force, but Russell wants to be more careful. He asks Josie to go back undercover and find out how many Nightbloods they have in total.
Josie rejoins Clarke's friends, where Madi questions her sudden change of mind. Gaia, Bellamy and Jordan are also surprised that Clarke seems to be willing to tolerate the Primes' actions. Gaia calls the Primes out for abusing Becca's Flame technology, whose intended purpose was to pass wisdom from one person to another. Instead, the Primes want to keep all the wisdom to themselves. Murphy disagrees with her, saying that Becca was just a scientist who was turned into a Nightblood in a lab - the same way Abby did to Clarke. This makes Josie realize that it is possible to turn ordinary people into Nightbloods. Josie gets up quickly to report the exciting news to Jade and her parents.
This makes Bellamy even more suspicious of "Clarke." He follows her and asks to speak to her alone. He notices that "Clarke" is having a hard time understanding Trigedasleng. To test her, he speaks in Trigedasleng explaining how he feels about all the people they've hurt, and how he sees them in his dreams. Unable to understand what Bellamy said, Josie asks him to repeat the last phrase. She then tries to figure it out by saying "flashpa" sounds like "flashback" which could mean "dream." At that point, Bellamy becomes convinced that that is not Clarke. He pins her against the wall and asks her who she is. Josephine knocks him out with a paralytic shot. As Bellamy lies on the ground, immobile, Josie reveals to him that she is Josephine Lightbourne.
With her cover almost blown, Josie decides to change her strategy, by going through Murphy. She gets Murphy drunk and makes it obvious that she's not Clarke by constantly calling him "John" instead of "Murphy" and looking at Josephine's picture. Murphy finally figures it out, but Josie convinces him to work with her by offering him immortality.
Meanwhile, in the woods, Octavia and Diyoza are chasing after Xavier - the remaining known member of the Children of Gabriel. Xavier leads them to a swamp-like ground called "the crucible" where they get stuck and begin to sink. He tells them that the more they fight it, the faster they sink. He offers to save them as long as they tell him who the other Royal Bloods are from Earth. Octavia asks Diyoza to tell them nothing.
While Diyoza remains calm, Octavia actively struggles to go free, which only causes her to sink faster. Diyoza asks Octavia to give up and tell Xavier, but Octavia would rather die. Diyoza asks Octavia if she really wants to die and offers to kill Octavia. She points a gun at Octavia's face but Octavia is not scared at all. She figures out that Octavia wants to die because people in her life hate her. Diyoza advises Octavia to stop caring about whether her friends hate her. Being hated by her friends pales in comparison to Diyoza's situation where her face is placed next to the worst people of all time.
Just as Octavia is about to completely sink, they notice a bright flare in the sky. Xavier says it's a temporal flare and it's very dangerous. He throws them a rope to escape and run to safety. However, Octavia is too submerged to move. To avoid slowing Diyoza down, she asks Diyoza to run and save her baby. Diyoza promises to come back for her. As the flare hits, Octavia closes her eyes and goes underground. Afterwards, Diyoza comes back to check on Octavia. She starts looking around and notices Octavia's body frozen underneath the ground. She breaks the hard surface layer and pulls Octavia out. Luckily, Octavia regains consciousness. Diyoza concludes that Octavia went underground for cover from the flare because she didn't really want to die despite acting like it. Octavia and Diyoza resume their hunt for the Children of Gabriel and their leader.