Skaikru and the Grounders alike struggle to recover after Alie's control is broken, leaving many wounded -- and Clarke bearing the terrible secret about the future of the planet. As Abby attempts to save the king's life, Ice Nation makes a move for control of Polis.

As Sky Crew search for a way to save everyone from the impending radiation, King Roan plans for a challenge by a rival Grounder. Jasper finds a new lease on what's left of his life, and Bellamy leads a trip to Farm Station to recover a desperately needed machine -- only to find that the crashed station is not unoccupied.

Arkadia takes in Grounders already affected by radiation sickness, forcing Clarke and the others to consider a new plan from Jaha -- or watch hundreds die. Roan turns to Octavia and Indra for help when the Flame is stolen, and Murphy returns to Arkadia.

Abby leads a team to Alie's laboratory to try and learn the secret of Night Blood, but get more than they bargained for from an automated defense system. Kane is caught in the middle when Roan decides Skaikru is working against him, and Jasper makes a discovery.

As Octavia fights for her life Arkadia is forced to prepare for an Ice Nation assault, and Clarke rides out to entreat with Roan for peace. Abby's attempts to create Night Blood inside Becca's laboratory is complicated when Raven has a seizure, leading to a startling discovery.

Clark and Roan face a difficult road through Grounder territory to deliver much-needed fuel, while Raven struggles to learn how to land the orbiter in one piece. Arkadia struggles to suppress a lust for revenge directed against the man who destroyed their lifeboat.

As Clarke and Abby weigh the moral cost of the Nightblood experiments Emori tries to convince Murphy to flee with her, convinced that her life is viewed as expendable. Meanwhile Bellamy risks his life when a black rainstorm hits Arkadia, and a reckless and despondent Octavia finds shelter with Ilian.

Abby and Clarke's experiments reach a critical moment after a discovery forces another difficult choice, while Jaha and Kane travel to Polis on the suspicion that the Second Dawn shelter they found may have been a decoy.

As word of Jaha's discovery spreads the Grounder clans clash over access to the temple bunker, putting Clarke in the middle once again. While Raven experiences hallucinations of Becca, Jasper leads a group who have decided to embrace death -- and Octavia tries to find a different kind of life.

With the alliance in tatters the Grounder tribes prepare for a final fight for the bunker, with Octavia putting her life on the line to fight for Skaikru. But not everyone is willing to fight fair.

Bellamy fights to convince Jaha, Abby, and Clarke to let the other clans into the bunker, while Raven's hallucinations help her to hatch a new plan to save her life. Back in Arkadia, Monty deals with the reality of his friends' decision.

Tensions run high as the leaders of Skaikru decide who will survive inside the bunker -- or whether they will risk all their lives by fighting Octavia and the Grounder clans. Clarke and Bellamy attempt to rescue Raven before time runs out, but run into trouble on the road.

As the radiation wave strikes Raven, Clarke, and Bellamy lead a daring plan to escape into space on board Becca's rocket -- but must be willing to sacrifice everything to survive.