Jasper is thrilled when he finds his dad's favorite childhood toy, Marcel Mouse, at his grandmother's place. He's so afraid to lose the family treasure that he keeps Marcel on a string around his neck. But sometimes Jasper has to take off the little plastic mouse, and that leads to all sorts of trouble. First, Marcel goes missing from under Jasper's pillow, then he gets sent to the principal's office and disappears again! Then one day, something TERRIBLE happens, and it seems Marcel might be lost forever … Will Jasper ever see Marcel Mouse again?
