This world is ruled by the amount of "sins" each person has. At age 10, they are tested. With 1 sins, they are high class, with 6, the lowest class. Those with 0 are called saints and those with 7 sins seen as the devil incarnate, eliminated immediately. However, as time progress, society gave those with 7 sins a chance. Aki, a 7 sins carrier, is given a chance to prove to be able to change before graduation or else she will be taken. During this time, she had the happiest time of her life.

Pigeon.