Clarke finds herself trapped in the underground colony of survivors inside Mount Weather, where her friends are convinced they should stay. Bellamy tries to rescue Finn, now a captive of a Grounder survivor. Octavia relies on Lincoln after she is poisoned, and Raven and Murphy are near death inside the drop ship.

After a patrol returns injured Clarke confronts President Wallace, convinced that her people are being lied to. Abby is forced to perform an emergency surgery on Raven, while Octavia goes to great lengths to help Lincoln. On the remains of the orbital Ark, Jaha discovers another survivor.

Clarke joins forces with Anya in an attempt to escape from Mount Weather. Abby confesses to aiding a rogue operation, and Finn and Bellamy discover a lead on their missing friends. Meanwhile, Octavia appeals to Lincoln's people for help in finding him.

Jaha is rescued by a family living in the desert, while Clarke and Anya attempt to elude capture in the forest. Raven is frustrated by her new limitations, and Bellamy's group attempts to rescue an Ark survivor from a cliff-face.

After Clarke is reunited with her mother, she learns that Finn is on a dangerous mission to rescue her -- and launches a rescue mission of her own. Kane attempts to make peace with the Grounders, and Jasper agrees to participate in a risky experiment.

When an acid fog separates a scouting party Bellamy and Octavia go in search of a way into Mount Weather, where they find a familiar face. Jaha and Kane are forced to make an impossible decision, and Jasper and Monty learn the truth about their new friends' plans for the 47.

Abby is put in a difficult position when her predecessor arrives at Camp Jaha and tells everyone they have to leave. Clarke is called upon to try and help Lincoln recover from his terrifying condition, leading to a new plan to forge a peace with the Grounders.

The Grounders lay siege to the camp, demanding that Finn be handed over to them to pay for his crimes. As Abby determines to prepare for a fight Kane returns, and Clarke and Bellamy decide to take their friend's fate into their own hands.

En route to a burial ritual with the Lexa and the Grounders, Clarke is plagued by visions of the man she killed. An assassination attempt throws the new alliance into chaos, and inside Mount Weather Monty and the group come up with a plan to contact the outside.

Clarke and Lexa find themselves on the run from a dangerous predator. Kane has a difficult time integrating his soldiers and the Grounders' fighting force, and Bellamy and Lincoln journey to Mount Weather with a plan to infiltrate the facility.

Bellamy is subject to horrifying conditions after infiltrating the mountain, while an exasperated Jasper confronts President Wallace. After Clarke is nearly killed by a sniper the Ark ends up holding an enemy prisoner.

Jaha sets out with a group for the legendary City of Light and encounter a stranger in the wastelands. Bellamy must find a way into a restricted level of Mount Weather after the experiments on his friends resume, and Clarke makes a fateful decision in order to preserve their inside man.

Following Mount Weather's attack the survivors come under sniper fire, thrusting Octavia into a leadership role. Abby tries to help Kane stay alive inside the rubble, and in the mountain Jasper and the others try to fend off the soldiers -- but soon must rescue one of their own.

While Raven and Wick try to guide Bellamy in neutralizing Mount Weather's acid fog, tensions rise between Clarke and Lexa when Octavia learns their shared secret. Elsewhere, Jaha's expedition discovers that a minefield stands between them and the legendary City of Lights.

The Sky People and the Grounders initiate a complex plan to infiltrate Mount Weather and rescue their people, with everything depending on Raven and Wick cutting off the enemy's power supply. Inside, Bellamy and Jasper work to free the innocent from the control of a corrupt government.

In the wake of betrayal Clarke and Octavia manage to infiltrate the mountain, where they find Maya's life on the line and their own people in enemy hands. As Cage moves ahead with his plan, Clarke is faced with an impossible choice. Elsewhere Jaha's group comes under attack from the water, and Murphy makes an astonishing discovery.