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.
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.
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.
Having discovered that Jasper may still be alive, Clarke, Bellamy, Octavia, Finn and Monty set out on a mission to locate their friend and are shocked at what they find. Meanwhile, on the Ark, Abby is determined to get to Earth, and enlists Raven to craft an escape pod.
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.