When a greedy businessman discovers that there is gold underneath a dam, he decides to demolish the dam and claim the gold for himself, and tricks a witch's young daughter to help him in order to keep Bamse from stopping him.
Bamse's friends go on a quest to find the Thunderbell, the ingredient that makes Bamse strong. Meanwhile, Reinard pretends to be a good guy to supplant Bamse as the hero of the town and win the affections of Mickelina.
Croesus Vole finds gold in the beavers' dam. To demolish the dam and get the gold he tricks the witch's daughter Lova to conjure away Bamse. With Bamse gone, it's up to the children to help each other to stop Croesus - but to do that they have to get along.