In the weeks after Praimfaya left the surface desolate Clarke struggles to survive on her own. On the station above the survivors work to return to the ground -- as a ship full of convicts arrives on Clarke's doorstep.
After the destruction of the surface world Octavia and the survivors inside the bunker face ongoing tensions in the face of overpopulation -- made worse when a group of Skaikru commandeer the underground farm level and barricade themselves inside.
While defending their home from the newcomers Clarke is captured, coming face-to-face with the prisoners' ruthless leader. Bellamy and Raven lead their friends onto the Eligius IV mothership in orbit, where they discover a new threat to the people they love.
Kane enters Queen's gladiatorial arena, while on the surface Bellamy attempts to negotiate with Colonel Diyoza to unbury the bunker. Raven spars with Lt. Shaw on the ground as he attempts to hack into the Eligius computer systems.
After a betrayal Octavia leads Wonkru on a march toward Shallow Valley, though a dangerous desert stands in their way. While Diyoza prepares for war Raven and Murphy are tortured, and Abby and Kane try to prove their use to the enemy.
Diyoza attempts to divide Wonkru by offering them food and sanctuary, undermining Octavia's leadership. Bellamy and Octavia clash over Echo's presence, and Clarke learns that Madi's future is under threat.
Echo looks for a way to convince Diyoza to trust her, as Kane confronts Abby over her addiction. Back at Polis Madi tries to stay under the radar during her training sessions, and Clarke and Monty make an unsettling discovery about the Red Queen's battle plans.
Clarke aims to sabotage Octavia's war plans, putting Bellamy and Indra in an impossible position. Abby makes a plea to Raven for help curing the prisoners, and Murphy must come up with a new plan to rescue his friends.
As hope for a cure begins to emerge Murphy attempts to turn Diyoza's people against one another, unleashing a civil war in Shallow Valley -- while Wonkru also threatens to itself apart in the face of a leadership change.
Octavia weighs the fates of those who betrayed her, as Monty tries to convince her of another way. Meanwhile Abby grows increasingly desperate under McCreary's control, and Madi and Clarke clash over her role as the future Commander.
After learning that Clarke may have betrayed Wonkru's plan to McCreary, Echo leads a mission to sabotage an ambush of their friends. As Abby fights through withdrawal she remembers the impossible choice that the people inside the bunker had to make in order to survive.
Wonkru marches on the valley only to fall victim to a bloody ambush, leaving Octavia and Bellamy pinned down. Meanwhile McCreary plans a final assault, and Clarke goes to extreme lengths to protect Madi's life.
As McCreary deploys his final solution to end the war, Wonkru's new commander acts to take the valley without further mass casualties -- testing the quality of her leadership.
