High School Undercover!.Lupin steals a precious diamond with the unique power to change the personality of anyone who looks at it, but during his escape the jewel ends up in a boy's backpack. To get it back, Lupin sneaks into the boy's high school as a new professor, and then finds out that the diamond has been found by a group of teachers that are planning to use it to build a bomb and free themselves from a blackmailer.