Yoshino Takigawa is visiting his friend's family grave. After getting there, he encounters Evangeline Yamato, a woman in search for his friend, Mahiro Fuwa, who left a month ago to find his family's killer without any clues to who it might be. Evangeline informs Yoshino that two places where Mahiro has been spotted have been quarantined, due to an epidemic.

When Evangeline is questioning Yoshino at gun-point, Mahiro comes out of nowhere and swiftly saves him and can stop bullets in mid-air, claiming to have a deal with a sorceress; not just any, but the strongest sorceress, Hakaze Kusaribe, who promised to help find his family's killer with her magic so he can kill the killer himself.

After saving Yoshino, Mahiro informs him about the epidemic, a curse that turns living things into metal, claiming the Kusaribe clan plans to revive the Tree of Zetsuen that could destroy the world.

(taken from http://www.mangaupdates.com/series.ht...)