In a city where the children live separated from their parents, two siblings discover a dark secret - their parents may not wait for them up there among the skyscrapers, and the emerging city they live in is in practice a prison. They discover the Underworld where an uprising against the government is planned, and realize that they themselves all the time had the key out of prison but were fooled to leave it. Soon they understand that if the uprising against the Government succeeds, their parents are at risk. They decide to find their parents before the uprising starts. But in the pursuit of the parents the siblings are separated for the first time, and soon everything is at stake.