Clarke struggles to make sense of her bizarre surroundings. The survivors of the Ark face physical and moral dilemmas. Jaha makes a heroic decision.
Clarke demands answers from President Wallace, Kane interrogates one of the 100, Abby performs emergency surgery on Raven, and Octavia resorts to violence to find Lincoln.
When Clarke discovers a horrifying project being carried out in Mount Weather's medical ward, she joins forces with an unlikely ally. Meanwhile, Kane issues an order to have Abby brutally punished after she confesses to a crime.
After being betrayed, Clarke takes matters into her own hands. Meanwhile, Bellamy, Finn and Murphy race against time to save a stranger, and tensions rise between Raven and Wick.
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 and Octavia comes face-to-face with her worst nightmare.
Clarke comes up with a bold new plan to stop the Grounders' attack, leading Abby to a desperate decision. At Mount Weather, Jasper and Monty begin to question the motives of their new friends, while Dr. Tsing conducts a deadly experiment to determine how to make use of the captives.
Finn struggles with the aftermath of his actions, as flashbacks show his and Raven's relationship on the Ark. Meanwhile, Clarke returns to Camp Jaha with devastating news, and Abby gathers information from an unlikely source and prepares for a fight.
Raven is accused of poisoning a grounder following Finn's death and the start of the peace treaty.
Clarke and Lexa encounter a new enemy. Bellamy and Lincoln agree to work together to break into Mount Weather. Murphy helps Jaha confront his past. Indra makes Octavia an offer she can't refuse.
A new enemy puts lives in danger; Lincoln and Bellamy decide to break into Mount Weather together. However, not all goes to plan. Lincoln and Bellamy's attempt to get into Mount Weather results in brutal consequences.
Tensions between Clarke and Abby come to a head after Clarke crosses the line. Raven helps Bellamy navigate Mount Weather. Jaha and Murphy encounter a stranger who may not be what she seems.
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.
Lexa and Clarke get into a heated discussion during battle preparations; Bellamy races against time.
Wick and Raven are faced with a setback; Maya and Jasper witness a terrible act.
Clarke gets help from a source she did not except; Cage continues on his mission; Bellamy and the group continue their efforts to break free.