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.
Finn arranges a meeting with Clarke and the leader of the Grounders in order to bring peace, however a sceptical Clarke warns Bellamy, who along with Jasper and Raven follow them to the meeting with weapons. On the Ark, Diana takes over the exodus ship.
Murphy returns to the camp carrying a horrific virus created by the Grounders to weaken the camp. After learning the Grounders plan to attack the next day, Bellamy, Finn and Raven come up with a plan to delay them.
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.
Clarke is faced with a difficult choice and Raven helps Bellamy navigate Mount Weather as Dr. Tsing begins to harvest the 47, while Jaha and Murphy encounter a stranger on their journey to the City of Light.
Clarke begins to question Lexa's leadership decisions and Indra continues to push Octavia. Meanwhile, at Mount Weather, Jasper steps up into the role of leader, and Cage throws Maya into a life threatening situation.