Krølle Bølle lives safely underground on Bornholm with his family and all the other trolls. Life here is full of bat flights, cave parties—and rules, lots of rules. One rule in particular must never be broken: do not go up to the surface, because humans are dangerous. But Krølle Bølle is a troublemaker who doesn’t quite understand what all the rules are for. When a sudden rumbling comes from above, he sneaks up toward the human world to see what’s happening. His little sister Borra follows him, and what begins as an innocent mischief-filled outing for the two of them quickly turns into a grand adventure—full of danger, magic, and experiences greater than anything Krølle Bølle could ever have dreamed of.