It's been nearly 100 years since Earth was devastated by a nuclear apocalypse, with the only survivors being the 400 inhabitants of 12 international space stations that were in orbit at the time. Three generations later, the survivors number 4,000 - and resources are running out on their dying "Ark" (the 12 stations now linked together and repurposed to keep the survivors alive). Draconian measures including capital punishment and population control are the order of the day, as the leaders of the Ark take ruthless steps to ensure their future - including secretly exiling a group of 100 juvenile prisoners to the Earth's surface to test whether it's habitable. No one has set foot on the planet in nearly a century - until now. For the 100 on Earth, the alien planet they've never known is a mysterious realm that can be magical one moment and lethal the next.