Clarke wakes up locked up in her Ark cell inside Josephine's mind. The room's walls are painted with drawings of Clarke's memories. When she touches a memory, she gets a flashback of the memory. She notices a door and tries to get out. She finds her late father, Jake Griffin inside of the gas station in Shallow Valley and starts wondering if she's dead. Jake asks Clarke the last memory she remembers, she recalls being killed by Russell and Simone Lightbourne. As she gets upset, it starts raining, after which her dad tells her that she's the one controlling it. He tells her to listen carefully. She hears a heartbeat and figures out that she's still alive. Jake advises Clarke to go back and figure out what's going on.
Back inside her room, Clarke is visited by A.L.I.E. who explains that the room is a repository of Clarke's memories. A.L.I.E. says that the only reason Clarke is alive is because A.L.I.E's neural mesh is still inside Clarke's brain. However, she warns that Clarke won't survive for long once the Primes figure out how she saved Raven - by EMP'ing A.L.I.E's chip. Clarke gets a flashback of how they removed A.L.I.E's mesh from inside Raven. The Primes would do the same. And once A.L.I.E's mesh is removed from Clarke, she would ultimately die. So, A.L.I.E removes the memory from the wall and puts it inside an EMP chip and gives it to Clarke for safe keeping.
Clarke tells A.L.I.E that she's noticed a room she's never seen in her life. So, she goes to open it but a young woman comes out instead. Clarke recognizes her as Josephine Lightbourne. Josie confirms to Clarke that she took over her body but warns that the body is not enough for both of them. She reveals that the room Clarke is in is called a Mindspace; and she has her own Mindspace as well. Mindspaces are created by the brain when two minds are inside the same body as a self preservation to keep the minds separate so that the body doesn't die. However, since Clarke opened Josie's Mindscape, she's mixed the minds, which would lead to an accelerated brain deterioration. Josie says the last time she experienced a mindwipe fail was back when Gabriel was still experimenting on how to upload the entire consciousness.
Josie tells Clarke that if they don't do something, her body dies. She asks Clarke to tell her how she died so that she can fix it. They only have two options - either kill Clarke and save Josie and the body, or they both die soon after which Josie will eventually be brought back. Clarke refuses to tell Josie anything but Josie threatens to find the memory in her Mindspace when Clarke thinks about it. She notices the missing memory on Clarke's Mindspace wall. Josie figures out that Clarke is holding the memory and tries to go for it, leading to a fight between her and Clarke. It doesn't take long before Clarke kills Josie with the door to her cell on the Ark. Unfortunately, Josie comes back from her Mindspace and says she can't really die in the Mindspace because her Mind Drive backs up her consciousness.
Clarke runs away from Josie and finds herself in the Second Dawn Bunker where she is visited by Octavia in her Blodreina look. She tells Octavia that she wants to hide but Octavia points out that's all Clarke does - hide when the rest are fighting. She tells Clarke that Bellamy hasn't forgiven her for abandoning him to die. Clarke gets mad at Octavia but Octavia reveals that she's just a subconscious projection of Clarke. As Josie approaches, Clarke asks Octavia to help her but Octavia refuses. Josie laughs at Clarke for being hated by even her own projections.
Clarke goes to another room where she finds Maya lying on a hospital bed, with burns all over her body. Maya blames Clarke for killing her at Mount Weather, in the name of saving her people. Clarke tries to apologize to Maya but Maya refuses the apology. She mentions that it was Clarke's fault that Jasper died because Maya's death is what sent him off the deep end. Maya points out that Clarke does things but other people pay the price. She's killed more people than she's actually saved. Maya asks Clarke to just give control to Josie. At that point, Clarke realizes that since it's her Mindspace, she is in control and can make anything happen. So, she sends away the chip with the memory for safe keeping. When Josie arrives, Maya tells her that she hid the memory chip in the cave. Josie, Maya and Clarke had to the cave. Upon arriving, Josie is shocked when she finds Maya already inside. Maya puts on a shock collar on Josie and electrocutes her. Clarke tells Josie that she has no control here since it's Clarke's Mindspace. Josie tells Clarke to give up because she can't win but Clarke says she'll find a way to survive as she always does. Josie electrocutes herself to die so that her Mind Drive brings her back up.
Josie brings Russell to Clarke's Mindspace, where they split up and start hunting Clarke to kill her. While running after Clarke, Josie notices a room that Clarke is afraid of. She goes in and realizes that it leads to a forest. She finds a box with a lock. As she touches it, she gets flashbacks of some of Clarke's most painful losses like Lexa's death. Clarke had hidden the memory there because it is the one place she never wanted to visit. Josie starts trying to figure out the combination but Clarke arrives and tries to stop her. Josie understands that all that guilt and trauma is because of things Clarke did to try to save her people. But she wonders why Clarke doesn't want to save her people now. She reveals that Bellamy has made a deal with the Primes - Sanctum would help Bellamy's people build their own compound and in return Bellamy wouldn't retaliate against them. Clarke doesn't believe Bellamy would give up like that - but Josie confirms it by making her touch a book of her memories. After hearing that Bellamy has given up so that the Earth people could live in peace, Clarke agrees to give up the body. She opens the combination and gives Josie the EMP chip with the memory on how to remove Clarke's consciousness completely.
After giving up, Clarke is visited by Monty who asks her why she give up like that without a fight. After some persuasion, Clarke agrees but it's too late since she's already given Josie the memory. Monty tells Clarke that they can figure it out together. They go inside Josie's Mindspace and find that it's filled with books of Josie's memories. They start hunting for a memory to use against Josie. While Monty is trying to open a room locked by Josie, Clarke starts touching some of Josie's books to learn more about her. In one of the books, she gets a memory of Josephine in her previous host arguing with Kaylee. In the memory, Kaylee has poisoned Josephine's Jo Juice as revenge since Josie killed Kaylee's boyfriend, Isaac. Kaylee then pushes Josephine through the window to make it look like an accident. In another book, there's a memory of how Josephine killed Isaac. She had discovered that Isaac was a spy for the Children of Gabriel and was trying to rescue an infant from death in the Offering Grove. In these memories, Clarke realizes that Josephine doesn't value anyone who isn't a Nightblood. The Primes call those without the Nightblood gene "Nulls" and treat them as disposable. This makes Clarke realize that she didn't really save her people by handing them over to Josephine Prime.
Monty finally opens the room. Clarke gets in and finds a memory of Josephine back on Earth, just before she left for Eligius III's Mission Team Alpha. In the room, Josie is doing homework with her friend in a diner when a man comes in and starts harassing her to go out with him. When Josie refuses, he claims that Josie thinks she's too good for him. He takes out a gun, and after a little argument, he shoots himself. After seeing the memory, Clarke and Monty realize that's the memory Josie couldn't face. Suddenly, Josie enters the diner and starts fighting Clarke for coming in there against her wishes. She says that her deal to help Clarke's people is now off. Clarke and Josie starts strangling each other until Josie wakes up in real life, declaring that she's won.
Josie gets up and finds Bellamy, Russell and Miller discussing terms of the deal. Josie coughs and Bellamy turns to look at her. He notices Josie tapping her arm with her finger like in Morse code. Bellamy notes down the taps. Inside Josephine's head, Monty and Clarke have figured out a way to communicate with Bellamy through Morse code by turning on-and-off the Christmas lights inside Josie's Mindscape. After the meeting, Bellamy reveals to Miller that Clarke was communicating to him using Morse code. Clarke's message says "ALIVE". Bellamy realizes that Clarke is alive and vows to do whatever it takes to bring her back.