When Akira Fudou attempts to rescue Ryou Asuka from an attack by the demon Oysnas, both are inexplicably transported back in time. After passing through what Ryou calls a "Timeslip," the pair realizes that demons have been infiltrating the past to interfere with significant historical events. Akira and Ryou must prevent these demons from corrupting pivotal figures in human history before they change the course of humanity.