In the city of Koka in Shiga Prefecture — also known as a “ninja village” — a written copy of the original, famous book on the art of ninja, “Karinseiyo,” has been discovered at a local shrine.

Cowabunga, dudes! If we were still living in the ’80s or ’90s, the radical heroes in a half-shell who thwart criminals using the martial art of ninjutsu while chowing down on pizza would be all the ...