Casper Tock hates risks and is allergic to adventures. So it comes as a nasty shock to him one day when he hides from bullies in an ancient grandfather clock--and ends up finding an entire world inside.

Casper discovers that this sky kingdom of Rumblestar is one of the four Unmapped Kingdoms in charge of controlling the weather in Casper's own world. What's more, Casper is now stuck in Rumblestar, and the only person there who doesn't think he's a demon spy is a strange girl named Utterly, who hates rules and is allergic to behaving.

All Casper wants is to find his way home, but Rumblestar is in trouble. An evil harpy called Morg is trying to steal the magic of the Unmapped Kingdoms. And if these kingdoms fall, Casper's world will be destroyed as well.

And so, together with Utterly and her miniature dragon, Arlo, Casper embarks on a quest full of cloud giants, storm ogres, and drizzle hags. Can he, Utterly, and Arlo--the unlikeliest of heroes--save both their worlds from the clutches of Morg?