Rikka Nikaidō is a normal school girl whose life is thrown into chaos when her bus flips into a lake while on a school field trip. The accident kills everyone on board except for herself. Rikka is hospitalized and recovers. When she leaves the hospital she can't move on from the trauma of the accident because she is continuously attacked in bizarre ways involving water. Linked to these attacks is a girl named Rikka Mizuchi. Rikka wonders who saved her when all her classmates were killed and what the demands of the evil Rikka are.