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!