The narrative takes place in the future on the planet Amoi, which is governed by a supercomputer called Jupiter. Within a predominantly male human society, the light-haired elite are permitted to temporarily keep dark-haired "mongrels" as pets. Iason, a member of the elite, meets a mongrel named Riki in the slums and chooses to take him in. However, Iason retains Riki for a period longer than is socially acceptable, leading to widespread rumors regarding the nature of their relationship.