As night descends over Iga, a fresh threat from the Koga shadows closes in on the academy's guardians. After a fateful meeting, Shinobu's closest friend Aya falls under the sway of a strange parasite and begins to lose herself to its control. The same fate now threatens Lena, forcing the group to infiltrate the Koga Institute's hidden labs in search of the only antidote that can still save her.