Nintama Rantarou: Ninjutsu Gakuen to Nazo no Onna - Kore wa Jiken da yo! no Dan

While Rantaro and Fushikizo are out on an errand, Fushikizo spots a mysterious woman fleeing ninjas from Tasogaredoki Castle. He brings the injured stranger back to Ninjutsu Academy, only for an astonishing truth to emerge: she is Hieta Happosai, leader of the Dokutake ninjas, in disguise.

Happosai claims his own side turned against him after a dangerous assignment, but the circumstances are far from clear. As the students and teachers investigate, suspicions involving Dokutake and Tasogaredoki pull the whole academy into a tangled incident where disguises, rival ninjas, and hidden motives make it difficult to tell who is chasing whom.