This is a movie built entirely on the confidence that you'll laugh at three ninja kids bumbling through a heist they have no business running. The plot is a clothesline for gags: incompetence collides with actual stakes, and nobody ever gets serious about solving the problem. It costs nothing but your time, and it pays back in the kind of comedy that doesn't need to explain itself, just watch Rantarou and his crew fail upward through a castle full of people who should absolutely stop them, but don't, because the show trusts you to find the chaos funny.

An unidentified castle hires Rikichi Yamada to recover a secret medicine that Dokutake Castle has stolen. The Dokutake ninja have taken a load of saltpeter from Shinbei's father, hoping to turn it into a medicine that lets them fly and makes them super-ninjas. Danzou, whose father was carrying the goods, hides aboard and ends up at the Dokutake's secret base. Once Shinbei alerts the school, Yamada-sensei and Doi-sensei hurry to help, but the students decide to mount a rescue of their own, and soon they need saving too. Yamada-sensei's son Rikichi steps in to lend a hand, and before long everyone joins in as Rantarou and his friends try to stop Happousai. The film is a comedy built around this ninja caper.