Edo period, 1860s, Japan.
A young boy with a mechanical arm, Renya, encounters a mysterious ageless and immortal witch seeking partners for a new covenant.
This witch, named Reifū C.C., also comes alongside a band of foreign soldiers from the far-off land of Great Britain, seeking a lost girl to return home with.
Behind the British army stands a mysterious man with purple eyes and dark hair, with mysterious beings known as "knightmares" standing in opposition to Renya and his band...