This episode page has been compiled from the episode description at the NDR website.
Rumpel is fetching mud for his mud bath, but his bucket is filled with so much mud that he spills some as he go back and forth. Meanwhile, Pferd discovers a funny looking mud stain and Detective Wolle immediately appears to investigate. Wolle takes a closer look at the mud stain. He concludes that it must have been brought to where it is since it doesn't have any legs. Pferd finds 2 more mud stains on the ground. Wolle concludes that it's a mud track and that if they follow it they will discover who made them.
They follow the track to the store, where they see a shape around a bathtub at Rumpel's corner. Wolle panics and hides because he is convinced that the shape is a mud monster with a pointy head. Meanwhile, Rumpel remembers that he needs his bath brush and goes to fetch it.
Wolle and Pferd observe the mud monster from their hiding place. Suddenly, Pferd notices that the mud monster is changing its head shape. Wolle thinks that they will have to go in closer to solve the case.
As they come closer they notice that the mud monster is singing. They decide to go in even closer. Meanwhile, Rumpel prepares for the thing he likes the most about taking mud baths: diving into the mud.
Wolle and Pferd reache the bathtub only to discover that the monster has disappeared. Then Rumpel pops up and Wolle and Pferd are scared. They start to run around and end up running into each other. Wolle is worried about the mud monster knowing their names, but Pferd tells him that the monster is really Rumpel. Wolle declares another case solved