Kazuki Muto was transformed into a Buso Renkin by Tokiko after he was killed by a homunculus, a flesh-eating monster. Now, Kazuki and Tokiko are racing against time to find the creator of the homunculi before the one that is lodged at Tokiko's waist reaches her brain and transforms her into a homunculus. Unknown to them, the person they seek is a student at Kazuki's school.