Life has returned to normal, but Shinichi has not as his ties to Migi continue to change him. As those around him grow wary of this 'new' Shinichi, a Parasyte named Hideo Shimada emerges from the shadows and begins attending Shinichi's school.

The confrontation between Yuko and Hideo comes to a gruesome head when Hideo goes on a rampage throughout the school. With Satomi missing, Shinichi must hurry and find her before Hideo can get to her first.