Yoshino Takigawa, a typical teenager, is secretly involved with Aika, the younger sister of his best friend, Mahiro Fuwa. However, after Aika dies under mysterious circumstances, Mahiro vanishes, swearing to track down her killer and exact revenge for the murder of his cherished sister. Yoshino carries on with his daily routine and goes a month without hearing from Mahiro—until a strange girl holds him at gunpoint, and his best friend arrives just in time to rescue him. Yoshino discovers that Mahiro has recruited a witch named Hakaze Kusaribe to hunt down Aika's murderer. Yet, Hakaze has been exiled to a deserted island due to internal strife within her clan. Her brother, Samon, selfishly seeks to exploit the power of the Tree of Exodus, disregarding both his sister's objections and the looming danger to the world. With Hakaze sidelined, Yoshino and Mahiro must now assist her in saving the world, all while drawing nearer to the truth behind Aika's death.