Sound of Drop -fall into poison-

Nestled in the heart of Tokyo, the quaint little aquarium, Manten Aquarium.
Numerous urban legends have arisen from here.
On the night of the full moon the waters turn to blood and man faced fish lurk within.

Junior high school students Mayu and Himeno came to Manten Aquarium to investigate these urban legends.
In the beginning it was fun, but then.
They see a glimpse of Mayu’s missing littler sister, Mari.
The wheels of fate begin turning as they chase after Mari’s shadow.

--Before she realizes it, Mayu finds herself alone and lost in the crimson aquarium.

[From Steam]