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 Zip 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. However, he finds a bag of marbles hidden in the room, and forms the "Marble gang" with several other students in the school, who are Filo, Micro, and Matilda, who is Falconetti's niece, and do various pranks, such as vandalising the statue of Sebastian Hope, the school's founder, and they always leave a single marble at their prank.