100 years in the future, when the Earth has been abandoned due to radioactivity, the last surviving humans live on an ark orbiting the planet — but the ark won’t last forever. So the repressive regime picks 100 expendable juvenile delinquents to send down to Earth to see if the planet is still habitable.
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 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 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.
Clarke is still on her own in the white room, frantically attempting to make sense of her bizarre surroundings. Lincoln and Octavia's plan is ruined and an enemy returns.
Kane takes part in a mission to make peace with the Grounders. Jasper decides to take part in a risky experiment. Lincoln enters a world of pain and President Dante Wallace issues a warning.
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.
Three months have passed since the tragedy at Mount Weather and our heroes learn that a bounty has been put on Clarke’s head. Unbeknownst to Clarke, a team led by Bellamy and Kane journeys deep into Grounder territory to save her. Meanwhile, Murphy finds his way to the mansion and discovers a very different Jaha on a very different mission.
Bellamy and Kane risk life and limb to rescue Clarke. In an attempt to avoid worrying about her daughter, Abby focuses on whether to open Mount Weather as a medical facility. Meanwhile, Murphy plots to betray his former Chancellor.
Lexa tries to maintain peace and order within the clans, and Clarke uncovers a strange and game-changing truth. Meanwhile, a flashback takes us into the darker chapter of humanity’s past.
Kane is searching for a peaceful way to handle things but soon realizes he may be forced to resort to drastic measures. Meanwhile, Pike suspects that there may be a leak within the walls of Arkadia. Lastly, Raven has a plan and reaches out to Jasper for help.
Clarke discovers a shocking truth, and Octavia is forced to make a decision that could have devastating consequences. Meanwhile, Kane and Abby share a moment.
Jaha employs a terrifying new method to convince Abby to join his cause, Kane is on a mission, and Bellamy is hit with a hard truth. Meanwhile, Monty finds himself in a precarious situation, and Jasper rushes to save one of his own.
Alie forces Raven’s dark side to come out, and Jasper’s anger continues to grow. Meanwhile, Octavia wrestles with her place in the world, and Monty is forced to make a split second decision.
Clarke is on a mission that could change everything. Meanwhile, Murphy gains insight that might prove useful in ensuring his survival. Lastly, Kane reaches a breaking point.
Continued struggles with leadership and trust in Arkadia and Polis take violent turns while Abby leads a team to a distant location hoping to find answers.
Kane's attempt to prove himself useful tests Wonkru's allegiance to Octavia. Madi faces an unexpected threat inside Wonkru, forcing Clarke to make an unlikely ally.
The growing fractures in Wonkru threaten to explode once and for all. Meanwhile, in Shadow Valley, Murphy starts a fire, unleashing catastrophic results.
Monty strives to show Wonkru an alternative to war, and to the valley itself. Meanwhile, Abby’s health continues to deteriorate, along with McCreary’s patience.
As Clarke races to save Abby, she learns more about the trials and tribulations Wonkru faced in the bunker, and the impossible decisions they were forced to make in the dark year.
Clarke and her friends must risk everything to fight one last battle for survival, only to glimpse an even darker threat to the last living valley on Earth.
Still reeling after receiving Monty’s message, a small group goes down to explore the mysterious new planet. Back on the Mothership, several members of Wonkru face the consequences of their decisions.
The team on the ground flights to survive the threat they face on the new planet. Meanwhile, Raven must join forces with an unlikely ally to save everyone on board the Mothership.
Clarke tries to win over the leaders of Sanctum in order to let her people stay. Meanwhile, Bellamy, Echo and Octavia discover a new threat while on a mission to retrieve the transport ship.
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