Only the witch's arithmetic can help here! Who will ever believe 12-year-old Felix that he shrank the math teacher, hated by everyone, to a size of 15.3 centimeters? He doesn't even know how it happened! But now he has the problem on his hands, or rather in his jacket pocket. Because there she is, the math teacher, still scolding him. How can Felix get her back to her original size? In search of the cause of the transformation, Felix becomes bolder and lets himself be bossed around less and less - but above all, he stumbles upon an incredible story that is said to have happened 100 years ago. Exciting and full of situational comedy - it speaks to the soul of students!.