Three classmates who make their living (literally!) by acting as agents of the Z-Loan office and dispatching zombies - Chika Akatsuki, Shito Tachibana, and Michiru Kita - are hot on the trail of an elusive psychopath known as "The Butterfly." But as if serial killers and zombies weren't enough to worry about, poor Michiru's barely had a chace to get used to her new digs, her new life, and...What's this? A new member!? Chika's friend Shiba joins the teen trio as they solve riddle after riddle and get ever closer to the madman's lair.