When Gnasher coughs up Mrs. Creecher's saxophone, Sergeant Slipper sends him to obedience school. 

Dennis can’t figure out why Gnasher would ingest a musical instrument, and goes on a quest to figure it out! 

With the help of Screwtop’s “Chatterbox”, Dennis can understand everything in Beanotown – plants, animals, and even machines. But before he can rescue Gnasher, Dennis and the gang have to help a bunch of animals in Beanotown.

But can all the animals work together to free Gnasher from Walter's obedience school? And will Dennis solve the mystery of Gnasher eating the saxophone?