Nine days before present time, a young man named Ilian, under the influence of the chip, kills his brother in an attempt to convince his mother to receive the key. Immediately after, Clarke stops ALIE in the City of Light and she vanishes. Ilian's mother dies, after telling her son to avenge her.
In Polis, Abby and Kane have sex before she must leave to help Roan. While dressing, she feels uncomfortable putting her wedding ring back on in front of Kane, but Kane assures her that Jake (her former husband) is a part of who she is. He warns her to be careful, since they are outsiders in Polis.
Ilian discusses with Rafael, the new ambassador of his clan (Trishanakru), why King Roan is protecting Skaikru. The ambassador vows to challenge Roan to single combat for revenge and in order to have their clan control Polis. He vows that Skaikru will fall with the king. Octavia eavesdrops, in disguise.
In the throne room, Echo questions Roan about why he is ruling like Lexa did and if there is something she doesn't know. Before Roan can say anything, Octavia and Kane enter the throne room. Roan kicks Echo out and Octavia and Kane tell the king that Rafael is going to challenge him. Octavia tries to offer to to fight in his place, but he refuses, saying his people won't accept a king who refuses to fight. They compromise, with Kane attempting to reason with Rafael before Roan is forced to fight.
At Arkadia, Raven tries to convince Bellamy and Clarke that the best bet for their survival is to find a place to ride out the radiation, since the meltdowns started months ago and they can't stop it. She believes they should tell everyone, to crowdsource for ideas. Bellamy and Clarke are hesitant to do that, believing it will start a panic. Monty comes up with a plan -- to repair the Ark, which kept them alive in space.
Bellamy goes to see Bryan and Miller, explaining that they need to go to Farm Station (in Ice Nation territory) in order to get the hydrogenerator that is needed to generate the water for their 500 people to survive in the Ark. Miller at first refuses, citing the danger, but Bryan insists on going. There is tension between them because of Bryan's allegiance to Pike.
Monty tells Jasper about the plan to retrieve the hydrogenerator. Jasper declines going along, happy enough to die soon enough, but asks Monty to retrieve the weed they stashed behind his wall. Monty agrees, and he, Bellamy, Harper, Bryan, and Miller leave for Farm Station. Raven and Clarke remain behind in Arkadia to look for a solution to save the grounders too.
The Farm Station group finds the station surrounded by Ice Nation on all sides. Bryan tries to suggest they kill all the Ice Nation warriors, but Bellamy says they're not there to fight. Suddenly, they're ambushed by warriors who take them captive and refuse to acknowledge the token Roan gave them for safety. They take the group to their chief, who begrudgingly releases them and takes them to the machine.
Raven instructs a small group of volunteers on tasks needed to repair the ship but is upset because she doesn't have enough people. Clarke promises that once Bellamy brings back the machine, they'll go public with what's happening. Jaha volunteers his assistance (having been an engineer before being Chancellor), but Raven is still angry at him for his actions on the Ark and while chipped and tells him to go sort the scrap.
Bellamy's group is taken to the machine, where they discover that the survivors from Farm Station were turned into slaves. Bryan recognizes a young man named Riley, who he thought was dead. The Ice Nation chief refuses to allow them to interact with the slaves.
Roan trains with Echo in the throne room, trying to prepare for combat with Rafael, but Abby tells him that he needs more time to heal. Roan refuses to back down from the fight. Echo bests him in training. When Abby leaves the room, Echo offers to kill Wanheda. Roan confides in Echo that the world is ending and Skaikru is attempting to save everyone. Echo believes they are lying and asks to go to Arkadia to find proof. Roan agrees to let her go after she helps him win the fight with Rafael.
Kane approaches Rafael and Ilian with Octavia and asks Rafael to reconsider challenging Roan. Rafael refuses to back down.
In Arkadia, Clarke becomes frustrated when she lists out all of the numbers in the various clans and can't figure out a way to protect everyone. She hears music outside and finds people lounging around not working. She confronts Jasper, who challenges her to be honest with their people and let them choose how to spend their last months -- enjoying them or fighting to live.
At Farm Station, Bryan tries to convince the group that they need to free their enslaved people. Harper and Bellamy initially say they'll try diplomacy first and ask Roan to have them freed, but they then find out that Ice Nation is moving the slaves and it's now or never -- they can destroy the hydrogenerator to use it as a bomb, but that would mean there's no way to provide water for the people on the ark. Bryan and Harper vote to destroy it to save the 25 slaves, while Miller and Monty vote to leave the slaves behind. Bellamy casts the tie-breaking vote, choosing to destroy the machine and save the slaves.
They incapacitate the chief and Bryan informs Monty that the chief is the man who killed his father, offering to let Monty kill him. Instead, Monty releases the slaves and allows them to kill the chief.
Jaha calls Clarke out on the truth about what they're doing with the repairs. She asks what to do and he advises her of what he advised her mother after the culling -- she must make the best decision she can with the information she has and then hope for forgiveness.
Back at Polis, Abby says farewell to Kane to go back to Arkadia, knowing that there's nothing more she can do for Roan in the capital.
Octavia sneaks in to see Rafael, offering him a last chance to change his mind about the fight. He refuses and she assassinates him swiftly with one stab to the head.
Roan attends an assembly of the ambassadors, also attended by Octavia. Roan notices Rafael is missing and Echo informs him that Rafael was found dead, with his heart stopped, which Roan announces to the assembled group. Kane seems to be aware of Octavia's interference, and Ilian asks her if she'll kill him too. She feigns ignorance and tells him she's sorry about his family.
Returning to Arkadia, the former slaves rejoin their people. Bryan can't forgive Miller for not thinking that saving the slaves was worth sacrificing the machine. Bellamy explains to Clarke and Raven the call that he made, which the two women aren't pleased about. Raven explains that without the hydrogenerator, the ship can only sustain 100 people.
Clarke agrees to crowdsource and tell her people what's up. But instead, she tells them only part of the truth, claiming that Alpha station can sustain all of them in order to compel them to volunteer to fix the ship. She vows that they'll survive and thrive together. Raven disapproves of Clarke's lie, but Bellamy understands her motivations.