Twin brothers Zip and Zap are sent to the summer school Hope, a re-education centre ran by strict Falconetti, who forbids all forms of recreation and entertainment. When Zap gets in trouble for disrupting a class, he is locked in the "Isolation Room" where he is told to think about what he has done. While there, he finds a bag of marbles hidden in the room, and forms the "Marble gang" with several other students in the school: Filo, Micro, and Matilda, who is Falconetti's niece; the gang performs a variety of pranks, including the vandalising of the statue of Sebastian Hope, the school's founder. They always leave a single marble at the site of their prank.
