In this world, there exists four different human races: regular humans called Niers, those who carry genes of beasts of the sky called Vamps, those who carry genes of beasts of the land called Lycans, and those who carry genes of beasts of the water called Sirens. The latter three are called Beasts of Blood (B.O.B.) Haruka is a simple Nier, the most populous of the races. He has grown up with his eccentric father - or rather, Haruka has grown up supporting his eccentric father and his spontaneous endeavors, his most recent surprise in store being marriage to a man with three kids of his own, all similar in age to Haruka. The only difference is that they are all B.O.B.s! Will the naive Nier Haruka survive his world being turned upside down!? Or will he be too dazzled by his new glamorous step-brothers...!?