The remnants of human civilization have escaped Earth to an orbiting space station after a nuclear holocaust devastated the planet. Ninety-seven years pass and resources are running out, so one hundred young prisoners are returned to the planet as guinea pigs to see if survival is possible. Back in space, a political power struggle is going on to see who controls mankind's destiny.
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.
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.