High above the clouds, a small grey Sweeper named Finnegan Fog has the most important job in the world: sweeping magical Sleep-Dust into the great silver Hopper that powers the night. Every evening, the Sandman collects it and sprinkles it over the world so every child, puppy, and teddy bear can drift into peaceful dreams. But one Tuesday, Finnegan discovers a hole in the workshop floor - and realises the Dust has been silently leaking away into the open sky for days. The Hopper is nearly empty. The Sandman is coming. And Finnegan, too proud and too panicked to ask for help, slides down a moonbeam to the world below to find more - not yet knowing that Sleep-Dust cannot be found at all. It can only be made.