Yaiba, accompanied by the 400-year-old Musashi Miyamoto, and his friends Sayaka, Kagetora, and Shonosuke, embarks on a training journey so he can defeat Onimaru. Onimaru, now a demon, has seized control of Japan and turned the parliament building into Onimaru Castle! His sights are set on Raijinken--Yaiba's Thunder God Sword--and he'll stop at nothing to take it!