Ninety-seven years ago, nuclear Armageddon decimated planet Earth, destroying civilization. The only survivors were the 400 inhabitants of 12 international space stations that were in orbit at the time. Three generations have been born in space, the survivors now number 4,000, and resources are running out on their dying "Ark." Among the 100 young exiles are Clarke, the bright teenage daughter of the Ark’s chief medical officer; the daredevil Finn; the brother/sister duo of Bellamy and Octavia, whose illegal sibling status has always led them to flaunt the rules, the lighthearted Jasper and the resourceful Monty. Technologically blind to what’s happening on the planet below them, the Ark’s leaders — Clarke’s widowed mother, Abby; Chancellor Jaha; and his shadowy second in command, Kane — are faced with difficult decisions about life, death and the continued existence of the human race.
In a desperate attempt to treat Jasper’s festering wounds, Clarke, Finn and Wells set out in search of a seaweed antibiotic. Bellamy and his crew go hunting for food, and are joined by Charlotte, a lovable 13-year-old, who feels safe under Bellamy’s protection. A thick, acidic fog appears, forcing Clarke, Finn and Wells to seek shelter inside an ancient van. While they are trapped, Clarke confronts Wells on his betrayal that resulted in her father’s death.
After the shocking death of one of their own, Clarke and Finn grow closer as they try to figure out a way to communicate with the Ark. Bellamy must keep the group from turning on one another as life on Earth takes a desperate turn. Meanwhile, Abby risks having herself floated in order to give Raven the chance to stow-away in the escape pod bound for Earth.
Clarke’s newfound romance with Finn is threatened. Octavia suffers a blow to the head and wakes up to find a Grounder watching her. On the Ark, Abby exposes Kane’s plan to reduce the population of the Ark. Jaha and Abby notice a signal and realize at least some of the 100 must still be alive on Earth.
Clarke and Raven make contact with the Ark, and Abby guides Clarke through a dangerous procedure in a desperate move to save one of their own. Meanwhile, on the Ark, Abby is removed from the council, and former Chancellor Diana Sydney takes her place. Jaha reveals to Abby that there are not enough drop ships for everyone on the Ark to go down to Earth, and Kane begins to deal with his guilt over his recent decision.
Clarke and Bellamy go on a mission to find supplies for approaching winter. Some of the 100 accidentally eat nuts with powerfully hallucinogenic properties and lose their grasp on reality. Meanwhile, Octavia seizes the opportunity to help a friend escape, and a terrifying event brings Clarke and Bellamy closer. On the Ark, Commander Shumway secretly assigns one of the 100 to kill one of his own, and the treacherous actions of Diane Sydney are revealed.
Murphy returns to camp claiming to have been held captive and tortured by the Grounders, and Clarke and Bellamy have very different opinions on how to handle his return. Octavia sneaks away to find Lincoln and returns with a serious warning for the 100. Meanwhile, a mysterious virus strikes camp forcing many to be quarantined, and Jasper pulls off a heroic move.
With a low food supply at camp, Clarke and Finn lead a group out to hunt for food and wind up in a compromising position. With Finn’s life in danger, Clarke makes a game-changing decision. Meanwhile, Raven makes Bellamy an offer he can’t refuse. On the Ark – Kane risks his life and is reunited with Chancellor Jaha and Abby.
Clarke is still alone in the white room, frantically trying to make sense of her bizarre surroundings, while the fate of Bellamy, Finn and Raven is still unknown. Lincoln and Octavia’s daring plan is thwarted and an enemy returns. Meanwhile, Abby, Kane and the survivors of the Ark must face physical and moral dilemmas in their dangerous and beautiful new world. Finally, Jaha makes a heroic decision.
When Clarke discovers a horrifying project being carried out in Mount Weather’s medical ward, she joins forces with an unlikely ally. After Abby confesses to committing a crime, Kane issues an order to have her brutally punished. Meanwhile, Monty warns a distracted Jasper that Clarke could be in trouble, and Octavia continues to fight for Lincoln.
After being betrayed by someone she thought she could trust, Clarke takes matters into her own hands and proves she’s not going down without a flight. Bellamy, Finn and Murphy race against time to save a stranger. Meanwhile, tensions rise between Raven and Wick while working together, and an unexpected reunion occurs.
Tensions run high between Clarke and Finn, while Raven discovers that Mount Weather is interfering with their 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. Finally, President Dante Wallace is about to be betrayed by those closest to him.
Clarke strikes a deal with Lexa and Indra. Abby and Kane begin to realize that decision making powers are out of their hands. A seething Raven is unable to control her emotions. A bold move threatens to destroy the newly formed alliance between the sky people and the Grounders, and a familiar face haunts Clarke. Meanwhile, back at Mount Weather, Monty comes up with a risky plan.
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. Octavia is prepared to fight for someone she loves. Meanwhile, things reach a breaking point at Mount Weather.
Clarke and Lexa prepare for battle, while Raven and Wick experience a major setback. Meanwhile, Octavia and Lincoln are each forced to make a difficult decision. At Mount Weather, Jasper and Maya witness a horrible act, Bellamy follows through on a promise and Cage goes public with powerful information.
The journey to the City of Light continues, and Jaha makes a move that shocks Murphy. Clarke receives help from an unexpected source, a visitor surprises Octavia, and Lincoln gets his revenge. Meanwhile, at Mount Weather, Cage moves forward with his mission and Bellamy and the group struggle to break free.
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.
Mysterious outsiders arrive with news of Clarke's missing people.