Yuhee is an ordinary girl who attends a private school. While waiting for her school bus, she picked up a bright purple conch shell on the beach. She fell asleep on the bus due to unbearable drowsiness. What unfolded in front of her when she woke up was a school that had sunk into the deep sea and an abyss of followers following the high priest. She solves puzzles prepared by Abyss Followers, avoids terrible monsters, and begins a journey to escape this Abyss School and return to her original world.