Discovering that Jasper may still be alive, Clarke, Bellamy, Finn, Wells and Murphy head out to find him. On the Ark, Abby is determined to prove Earth is habitable, and enlists a mechanic to craft an escape pod.
As a dangerous, acidic fog approaches, Clarke, Finn and Wells set out in search of a seaweed-like plant to make an antibiotic poultice for Jasper's wounds, while a vulnerable young girl follows when Bellamy takes a group out hunting for food.
Bellamy, Clarke and Finn try to protect Charlotte when everyone learns she killed Wells. On the Ark, Abby risks getting floated in order to give Raven the chance to launch the escape pod.
Bellamy, Finn and Jasper head into Grounder territory to search for Octavia, as flashbacks show the siblings' difficult childhood on the Ark. Meanwhile, Raven finds out about Clarke and Finn.
While Bellamy tortures the Grounder, Raven and Clarke make contact with the Ark and ask Abby to help them save Finn. On the Ark, Kane deals with his guilt, and Jaha surprises the council with a shocking revelation.
Shumway secretly orders one of the 100 to kill Bellamy. Meanwhile, hallucinogenic nuts provide Octavia with an opportunity to help the Grounder escape the camp, and Bellamy and Clarke investigate a nearby underground depot, where they find military weapons.
Finn organiza una reunión entre Clarke y el líder de los Terrícolas con el objetivo de encontrar la paz, sin embargo, esta avisa a Bellamy, quien junto con Jasper y Raven les siguen. En el Arca, Diana toma el control de la nave del éxodo.
Murphy vuelve al campamento portando un terrible virus creado por los Terrícolas para debilitar al campamento. Después de enterarse que estos atacaran al día siguiente, Bellamy, Finn y Raven idean un plan para retrasarlos.
Clarke and Finn are captured by Anya, who forces them to save the life of a victim of the explosion. Meanwhile, Raven deals with her split from Finn, and Kane fights to save the Ark and it's survivors.
Clarke and Finn come face to face with a new enemy after Lincoln rescues them from Anya, while Bellamy, Raven, Octavia and Jasper deal with a vengeful Murphy. On the Ark, Jaha makes a plan to try and get to Earth.
