With bizarre schoolmate Seine now living at his house, things around Tatsuya only get weirder. Seine is waging war against evil mermaids with the ability to control humans, and only she has the power to destroy them with the help of her snake-like creature named AiON. Meanwhile, Tatsuya's childhood friend Nagisa is apprehensive about Tatsuya being too nice to Seine. But when mysterious twins show up calling themselves Seine's guardians, will Tatsuya be able to keep his friends?