At the start of The 100 Season 2 Episode 12, the Mountain Man prisoner Clarke freed at the end of The 100 Season 2 Episode 11 is running out of oxygen. Cage, the new President Wallace, noticeably missing any protective gear, finds him just in time and tells him that the ground is theirs.
Meanwhile, inside Mount Weather, Jasper tells the others that he saw Bellamy, but before they have time to process this turn of events, Dr. Tsing comes to collect the next candidate for bone marrow extraction. Jasper puts up a fight, but it's not enough to stop her.
Tensions are also high at the Ark, where Clarke is nervous about not being prepared for the upcoming fight. She butts heads with Raven over their progress. Raven is doing the best she can, but they need more information from Bellamy on the acid fog in order to disable it.
The 12 Grounder clans are meeting at Ton DC and Clarke is suppose to go, but when Bellamy radios and informs them about the latest snafu at Mount Weather, Clarke decides to send Kane in her place.
While war is on everyone's mind at Camp Jaha, actual Jaha is leading a group through the desert in hopes to find the City of Light.
Instead of the City, they come upon a woman Emori, who tells them that she was traveling with her brother but "Wastelanders" got to them and killed him. Jaha and Murphy offer her water and ask her to join their group.
At Mount Weather, Bellamy is in the ducts talking to Raven and Clarke via walkie. They overhear Cage talking about sending a missile to take out the Grounder summit at Ton DC. To keep him thinking clearly, Clarke lies to Bellamy about Octavia's whereabouts.
She tells him O is at Camp Jaha, when she is really already at Ton DC with Indra. O is worried about Lincoln, who hasn't been back to camp. Indra tells her that she can't worry about it until after the war.
In the Dead Zone, Murphy and Emori bond over the skeletons in their closets. Murphy admits that he has killed people, while Emori reveals a birth defect caused by the radiation.
At the Mountain, Dr. Tsing has come to take another of the 100. Bellamy disguised as security approaches Jasper and slips him a gun.
Bellamy, still dressed as a guard, brings dinner to the former President Wallace, tells him Jasper sent him and asks for help. The older Wallace says that while he can't help them escape, he may be able to buy them some time.
Clarke is racing against time to get to Ton DC. When she arrives, she tells Lexa about the missile in private. Lexa argues that they must let it happen as not to alert the Mountain Men to Bellamy's infiltration. Clarke fights it, but ultimately sees her point.
In the desert, Emori double crosses the Arkers, steals their supplies and holds Murphy at knife point.
At Camp Jaha, Raven is guiding Bellamy over the radio and lets it slip that Octavia is at Ton DC. While he is worried for his sister's safety, he knows that he must continue his mission.
Back at Ton DC, Octavia is patrolling and comes across Lincoln in the woods. He is not full Reaper yet. He admits to her that he is weak and betrayed her brother. She speaks the Grounder language and tells him to fight it.
When Murphy wakes up in the desert, he tells Jaha that Emori told him that the City of Light was due north. Murphy and Jaha choose to continue the journey with no supplies and only minimal directions.
Lexa and Clarke are on their way out of dodge, but Clarke is still having second thoughts. When she sees her mother, she instinctively runs to save her. She manages to convince Abby to come with her, but when the missile strikes and Ton DC is annihilated, Abby realizes that her daughter knew what was going to happen.
Before the explosion Kane and Indra had been on their way to find Clarke and Lexa. It's too soon to tell if they made it out of Ton DC in time.