With Marco defeated and Zeus threatening to use the terrifying power of Medusa to re-create the world, it's up to Kasumi to summon the courage to fight back.
After narrowly escaping from their island prison into a half-submerged tunnel, Kasumi and her sister, Shizuku, soon find that evil lurks in the inky depths with a mysterious girl watching their every move, and monstrosities waiting to devour them.