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.

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.

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.

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.

As the remaining members of the 100 face off against the Grounders, Jaha makes a noble sacrifice to ensure the Ark makes it 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.

Clarke wants answers from the President. Abby performs emergency surgery. Octavia will do anything to find Lincoln.

After discovering a horrifying project in Mount Weather's medical ward, Clarke joins forces with an unexpected ally.

Clarke ends up being betrayed by somebody that she trusted. Bellamy, Finn and Murphy attempt to save a stranger.

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.

Tensions are running high between Clarke and Finn. Raven finds out that Mount Weather is interfering with their communication systems.

Bellamy is able to get Clarke to agree to accompany him to the drop ship. Dr. Tsing is in charge of a deadly experiment.

Clarke comes back to Camp Jaha with some shocking news. Finn has to face the consequences of his actions.

Lexa and Clarke manage to make a deal.The newly created alliance is under threat. Monty comes up with a plan.

Lives are left in danger following the arrival of a new threat. Lincoln and Bellamy decide to break into Mount Weather.

Lincoln and Bellamy's efforts to get into Mount Weather has some brutal consequences.

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.

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.

Jaha makes a surprising move. Clarke gets help from an unexpected source. Lincoln seeks revenge.

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.

Clarke wrestles with a decision. Bellamy learns that something isn't what it seems. Meanwhile, Murphy plots to betray his former Chancellor.

Clarke discovers the mastermind behind a devious plan. Kane struggles to keep the peace. Meanwhile, Jasper’s grief drives him to reckless behavior.

Pike's twisted crusade threatens Clarke's attempts for peace. Raven becomes a target, while Murphy tries to conspire against Jaha.

Clarke is torn between vengeance and mercy. Meanwhile, Kane and Octavia work together to avert a disaster, and Abby continues to worry about Raven.

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.

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

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.

Clarke, Bellamy, Octavia and Jasper hit a tragic roadblock. Meanwhile, Raven and Monty make an important discovery.

Clarke finds hope in the most unlikely place. Meanwhile, Alie’s master plan comes together.

Our heroes begin to face the reality of their tragic situation. Everyone prepares for a final showdown.