General Yoroi's Snailbura implants its sinister eggs into the residents of an apartment complex. These incubate in the victims' bloodstream developing into larvae to enslave them. Snailbura's first experiment is ordered to kill Kazami Shiro, but dies before she can complete her mission. Her sister Misako accuses Shiro of murder and Snailbura promises to give her the power to avenge her sibling's death.