One in every ten person develops a magical ability during their teenage years, but only some of them gain abilities powerful enough to be of any use. The phenomenon is called double-life, their capacity is called second-life and one with double-life is called a doubler. A person who has not develop a double-life is called — by the doubler — a singler. Doublers are forced to live on Effingdurn where they either live isolated or learn to become a monster hunter/mercenaries.