The mind of Ritsuka Fujimaru, a Master from Chaldea, has been whisked away to Edo-period Japan. But she soon realizes that things don't quite match up with Edo-period history from her timeline, and after she reunites with Lady Miyamoto Musashi, she finds herself embroiled in a fierce battle against the Heroic Spirit Swordmasters, seven cursed undead warriors who leave annihilation in their wake.
Musashi and Ritsuka defeat the second of the Swordmasters in battle, but they scarcely have a moment's time to reflect on their victory before the third Swordmaster makes her presence known. She is a shinobi whose target is Kiyohime, the princess of Toke Castle in Shimousa Province. She conjures great serpents and strikes under cover of night...but Ritsuka has a ninja of her own on her side: Kotarou, who's mounted a full recovery after Rayshifting from Chaldea to join his Master.