Nelly Rapp has learned to handle monsters and other dangerous beings with the help of the three C’s: calm, cleverness and cunning. But are Nelly and Valle talented enough to stand up to the director of the School of the Arts, who is threatening to close the school if they don’t play well enough?

Nelly and Valle are back at the Monster Academy to learn about monsters and other dangerous beings and the subject this time is the Nixie: a cunning water creature said to play music to lure children into his waters and to their deaths. But nobody knows for sure if he exists as he hasn’t been seen for over a hundred years.

During recess, as Nelly and Valle are walking their dogs in the forest, they spot a man in the middle of a stream. Could it be the Nixie? No, it’s just Uncle Hannibal trying to find the Nixie in order to get a professional violin teacher. That reminds Nelly that she has to rush back to town in order to practice the cello, because the director of the School of the Arts has threatened to close down the school if anyone plays out of tune at the spring concert. And considering how it sounded last time, they definitely need a teacher as talented as the Nixie to whip them into shape…