The story follows Kakujou Youkou, who lives in a seemingly normal world where medical technology has skyrocketed to new heights, making it possible to completely replace any human organ. Those behind this are called the Ame No Tori (Rain of Birds), but it all turns out to be a plot against humanity where the new technology can take over its host, thus turning them into a killing machine. A task force is created called the Jackalope; its goal is to annihilate these mindless murderers before they raise any more havoc. This is where Youkou's new life begins...