In Polis, a young woman named Gaia -- the same one who voiced her displeasure during Roan's speech in the season premiere when he said he would be holding onto the flame until another Nightblood ascends to the Commandership -- steals the flame from Roan. Roan recruits Octavia to track the flame down, explaining his theory that Gaia, the new flamekeeper, in the aftermath of Titus' death.
Octavia, with Indra, finds Gaia, who is a devotee and finds the flame sacred. Gaia refuses to give up the flame. It is revealed that Gaia is Indra’s daughter and that Gaia left home, disappointing her mother with her flamekeeper lifestyle choice and non-violent beliefs. In the end, Octavia and Indra help Gaia to escape with the flame, handing over Gaia's necklace (which looks similar to the flame). The necklace is desroyed and Gaia gets out with the flame intact.
Murphy, who is living out in the wild with Emori and scavenging for food, convinces her that he must go to Arkadia to steal supplies for them.
Meanwhile in Arkadia, Clarke and the Sky People are still looking for a way to save everyone from the impending radiation when Luna and a handful of her people (accompanied by Nyko) show up on their doorstep. Luna and her people on the oil rig were among the first in their area hit by Acute Radiation Syndrome. All but Luna, the little girl she's close with, and a few others are dead from the radiation, while the rest are slowly succumbing to their injuries.
Abby works to save Luna and her people. She goes to Raven to ask for medicine to use in order to attempt to save the little Grounder girl. Because the cure isn't guaranteed to save her -- and the Sky People need to ration that medication to remain healthy when the radiation comes -- Raven refuses to hand over any of the medication. Murphy, who overhears their conversation, puts his thieving skills to good use, stealing the medication for Abby to give to the girl. Unfortunately, the medication doesn't work and the girl (along with the other Grounders except Luna) dies.
While Abby and Jackson are fighting Raven and working to save Luna's group, Bellamy, Clarke, and Jaha decide to go off in search of a rumored fallout shelter that was created by a cult called The Four Horsemen before the first apocalypse. Raven is at first against them going -- the supposed shelter is a day's drive, and Raven needs the truck for her repairs.
Clarke convinces Raven to allow them to use the truck, given that if the shelter is there, they can save many more people than Arkadia could feasibly house. When they get to the shelter site, they do find it is there but also find that the cult was a sham -- the fallout shelter (covered in angry graffiti) was not secured, and everyone who locked themselves in there died horribly when the radiation hit. The trio head back to Arkadia, feeling defeated.
Murphy, meanwhile, returns to the cave to retrieve Emori. Since he overheard the end of the world situation, he convinces her that she needs to come with him back to Arkadia so they can insure their own survival when the radiation hits. He explains that he's already started on convincing Abby that they should survive.
As she agreed when Raven allowed them to take the truck, Clarke sits down to make the list of the 100 people that can be accommodated in Alpha station, assuming they cannot find an alternative way to protect everyone. When she gets to the end of the list and the remaining two spots, she puts Bellamy's name on. Bellamy sees her emotional over making these decisions, and adds Clarke's name to the list himself, insisting that if he's on there, she's on there.
Unexpectedly in the final moments, despite all her remaining people dying at Arkadia, Luna miraculously recovers from her ARS. Abby realizes the difference between Luna and the people who succumbed to the radiation poisoning -- Luna has nightblood and they do not. They realize that nightblood protects from/heals radiation posioning.