You live your lives, from birth to death, in the shadow of the Wall. More than thirty miles tall, and wrapping completely around the world. Everyone you know lives with just enough to survive - everything else goes to maintaining the Wall. It protects you all. It imprisons you. Everyone you know either works on the wall or has half of their family working on the wall. Why was it built? What was it protecting us from? No one living knows. No one can climb to the top without a supply of air, and no one who reaches the top is allowed to return. What is being hidden?
Nothing.
Literal nothing, the actual vacuum of space. Your planet is only a hemisphere, the rest of the world destroyed by a cataclysm that has been buried or lost to time. Now all that remains is the structure that keeps you alive, trapping the atmosphere within - the World-Wall. But it isn't enough. You are dying by inches as air and water seep through a billion billion tiny cracks. This won't keep everyone alive forever.
The party must discover the secret of the Wall. And then they have to save the world, in the most literal sense - either by finding a new home for all life, or by taking the remnants of the world they have and reforging them into something new. They are opposed by the world-nation which has arisen to control and maintain the Wall. They won't simply let go of their power.