The red sun derails Clarke's plans.

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.

97 years after a nuclear war, human kind is living in space. 100 juvenile delinquents are sent down to Earth to see if the planet is habitable.

Where in the universe is Bellamy Blake?

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.

Mysterious outsiders arrive with news of Clarke's missing people.

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.

Clarke and her friends find themselves on a planet that is not what it seems.

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.

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.

Jaha returns to Polis, and Murphy has a surprise encounter. Meanwhile, Octavia uncovers a clue.

Clarke and her friends struggle with how to proceed after the fate of the world is revealed.

Clarke and her friends attempt to rebuild Sanctum as a new threat rises in the woods.

A plan to contact the Ark is put in motion after Raven crash lands on Earth. Meanwhile, Abby exposes Kane's plan to reduce the population of the Ark.

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.

Jasper and Bellamy go on a quest. Meanwhile, Clarke struggles to keep the peace after word of Jaha's discovery spreads.

Echo and Gabriel learn more about Hope and her mysterious past.

Raven faces an unexpected threat. Clarke must keep the peace among opposing factions in Sanctum.

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.

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.

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.

The burden of leading weighs heavily upon Clarke and Bellamy when different challenges force them to determine who will live and die.

Octavia fights in the final battle for her people's survival, but not everyone is willing to play fair.