It wasn't a thing, it was a bit of shaped sky . . .
Somewhere in a place that is so far up there is no down, a ship is waiting to take the nomes home - back to wherever they came from. And one nome, Masklin, knows that they've got to try and contact this ship.
It means going to Florida (wherever that is), then getting to the launch of a communications satellite (whatever that is). A ridiculous plan. Impossible. But Masklin doesn't know this, so he tries to do it anyway. And the first step is to try and hitch a ride on a new kind of truck, a truck with wings - Concorde.
The final title in the magnificent trilogy, The Bromeliad.