Japan has become a wasteland devastated by the Rusty Wind which causes all things to rust away. A suspicious looking priest visits a checkpoint all alone, yet on his back rests a bow priests wouldn't use. Meanwhile, Milo, a doctor running a clinic in the town of Imihama, is researching new medicines in secret in order to save his older sister, Pawoo, who's being corroded by the Rust. But that day a stranger appears before him...

The wanted criminal, Akaboshi Bisco, causes havoc in the town of Imihama using his bow to cause giant mushrooms to bloom. Bisco searches for a doctor who can synthesize medicine to save his mentor, Jabi, who has become gravely ill. Bisco storms into the clinic and accosts Milo, but Governor Kurokawa hears the commotion and dispatches his elite forces.

Bisco and Jabi continue their journey through Touhoku in search of Sabikui, the Rust-Eater, a mushroom said to cleanse away any rust. Milo also realizes that if he could cure his sister, Pawoo, if he had this Rust-Eater. Meanwhile, the governor of Imihama, Kurokawa, burns with an unusual obsession with killing Bisco.

The revived Testujin advances, destroying everything in its path. If this goes on Japan will once again be swallowed up by Rust... After inheriting the spirit of Bisco, Milo resolutely takes a stand against Testujin. Jabi, Pawoo, and Tirol race to his aid and they manage to corner the rampaging Testujin, but...

Tetsujin is heading for Imihama, repeatedly attacking Milo in a fierce assault. Yet just as he's about to be killed, Bisco revives from within Tetsujin. Together Bisco and Milo join forces in the final battle against Tetsujin!