When Kazuki Azuma returns to Japan after 10 years, he gets assaulted by a group of armed men and becomes their prisoner. Asabuki Kogane, a childhood friend, saves him with a weapon on her right hand known as Bubuki; a weapon with its own mind. Kazuki Azuma, who is a Bubuki user himself, learns about the existence of Bubuki and goes on a journey alongside the companions he has found, in an attempt to find and revive Oumai, a Buranki (titan) who sleeps underground. Source: AnimeLab