Hissori and Gami were just two, normal teenage sisters who were born on the exact same date and time, just on different years. Who share the exact same birthmark just on different parts of their body. Nothing suspicious about that, right? Well, when the girls' mother suddenly dies on their birthday, they begin finding others with the same birthmark. And when they finally uncover the secret, they find out they're very special, and were created for a very important reason.