Nearly a century after a global catastrophe, a group of teenage delinquents are sent to the surface of Earth to determine if the planet is viable for recolonization -- while the oxygen on board the orbiting Arc dwindles dangerously low.
Clarke forms a rescue party when she discovers that Jasper may still be alive. Bellamy tries to shore-up support for his leadership, and on the Ark Abby must convince Chancellor Jaha not to begin culling the population.
Clarke, Finn, and Wells go in search of a seaweed they believe might save Jasper's life, but must seek refuge from a thick fog of acid. Meanwhile Bellamy gives council to a young girl struggling with nightmares.
Clarke discovers that her friend was killed by one of their own, inciting the camp into a murderous fury. On the Ark, Abby and Raven attempt a risky, black market negotiation for a part that will allow them to complete the drop ship more quickly.
Clarke and Finn race to beat Bellamy to Raven's pod after it crashes on the surface, offering the potential of contact with the Ark. Abby takes matters into her own hands when she learns that Chancellor Jaha and Kane plan to reduce the population through an act of mass murder.
Bellamy leads a search party into Grounder territory to find Octavia, who has been taken captive by the warrior tribe. Raven grows suspicious of Clarke's relationship with Finn, and in the past Bellamy learns how to protect his little sister.
While a hurricane bears down on the encampment, Raven tries desperately to make contact with the Ark so that Clarke can get help from her mother in saving Finn's life. Meanwhile Bellamy questions a captured Grounder, and on the Ark Jaha and Kane deal with the fallout of their recent decision.
With intel from the Ark, Bellamy and Clarke go in search of supplies to help the group survive the coming winter -- unaware of an assassin on their tail. Octavia makes a personal connection with the prisoner, and the camp is affected by a nut with hallucinogenic properties.
Finn seeks Clarke's help in negotiating a peace with the Grounders before the first drop ships arrive. On the Ark, the annual Unity Day celebration is disrupted by a deadly terrorist attack.
After Murphy returns to camp after being held and tortured by the Grounders, Clarke discovers that he has infected them with a deadly and highly infectious disease. Learning that the Grounders are planning a direct assault, Raven finds a way to make a major show of force at the bridge.
After the camp's food supply is lost Clarke and Finn lead a hunting party into the wood, only to find themselves in the hands of the Grounders. Kane and the other Ark survivors fight for their lives after the Ark is left crippled.
As the camp prepares for the coming war with the Grounders, Murphy seeks revenge and winds up taking a hostage inside the drop ship. Clarke and Finn face a death sentence from their captors, and the survivors on the Ark face the prospect of only a few days of air remaining.
An enemy attack forces the survivors of the 100 to dig in at the camp, with one desperate plan to turn the tide before they are all slaughtered. Finn risks his life to give Raven and the others a fighting chance, and plans to land the Ark are complicated.
