In class 3D, a theft has occurred! The students are in a state of excitement. When Lilibeth and Pik Ass start to speculate among themselves, the Thinker dismisses them. He wants to systematically get to the bottom of the matter. But when the missing golden watch is found in the bank of Sir, his friend, the Thinker must hurry. Could Sir be a thief? Never, believe Lilibeth and Pik Ass as well. However, to prove Sir's innocence, they must find the real thief - and that is primarily the Thinker's task.