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

Synopsis

While Rantarou and Fushikizou are out running an errand, Fushikizou happens to spot a mysterious woman who's being chased by some of the Tasogaredoki Castle ninjas. It turns out the woman is actually the disguised leader of the Dokutake Castle ninjas, Hieta Happousai, and the entire Ninjutsu Academy ends up getting involved in the events.