It's the fourth week of summer, and young detectives do nothing but hide from the heat in the shade of trees and fuss from boredom. There is not a single hint of mystery in Peterswood! In order to somehow unwind, the guys go to a sports festival and meet Inspector Jenks there, but he also has nothing to please the team of detectives. Fortunately, luck smiles on the guys: a thief is announced in Peterswood, but not a simple one, but an invisible one. Young detectives and a detective dog will not be frightened by this - they are sure that they will expose the mysterious criminal before the end of the summer holidays!