The protagonist is summoned to another world with his step-mother and two step-sisters. Demi-humans are the rulers and saviors here, fighting undead monsters in groups of four called a "familia". Each person takes one of four roles: the swordmaster, the guardian, the healer, and the black mage. But to use magic power, one must offer part of their being as a sacrifice - the higher the sacrifice, the higher the power. Now the stranded family must decide who will take each role and accept the power - and consequences - that come with it.