Clarke faces betrayal. Meanwhile, Bellamy, Finn, Murphy, Sterling and Monroe race against time to save a survivor of the Ark crash, and Raven struggles to cope with her paralysis.
While Kane leads a mission to make peace with the Grounders, Jasper agrees to participate in a risky experiment, and Finn's search for Clarke takes a violent turn.
Tensions run high between Clarke and Finn, and Raven discovers that Mount Weather is interfering with Camp Jaha's communication systems. Meanwhile, Jasper and Monty discover the truth about what's going on at Mount Weather.
As Abby and Jaha each try to establish their authority, Clarke comes up with a plan to stop the Grounders' attack. At Mount Weather, Jasper, Monty, Miller and Harper plan their escape, as Dr. Tsing and Cage conduct deadly experiments.
Clarke and Lexa encounter a new enemy. Bellamy and Lincoln work together to break into Mount Weather. Murphy helps Jaha confront his past. Indra makes Octavia an offer she can't refuse.
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.
As Jaha and Murphy encounter a perilous obstacle on their journey, Raven and Wick try to figure out how to disable the acid fog, and Bellamy races against time as his cover is blown.
After being betrayed by Lexa, Clarke makes a final stand against Mount Weather, while Jaha shocks Murphy as they continue their journey to the City of Light.