97 years after a nuclear apocalypse has devastated the surface of Earth, remaining humans live in orbiting space stations known as "The Ark". They send 100 juveniles convicted back to the surface to test its habitability. After surviving from the war with Grounders and Mount Weather, now, they return to a world seemingly at peace, but the peace doesn't last long since they soon have to face new challenges which test their loyalties and push them past their limits, and make them wonder what it truly means to be human.