Sasae Hatsushima and his family, stepmom Kanae,
stepsisters(aunts) Manako and Hinana are summoned to another
world to fight the Dead Mell, undead monsters tormenting
the northern half of the continent which can only be killed by using weapons made with holy silver.
In order to fight, Sasae and his family must form a familia,
a group of four people with them being the swordmaster that uses the holy sword,
the guardian that uses the power of earth to protect,
the healer that uses the power of water to heal,
and the black mage that uses the power of fire to destroy.
The role of the guardian, the mage and the healer is to support the swordmaster that can only kill the monsters.
But to use magic powers one must offer part of your being as a sacrifice to form a contract with spirits, and the higher the sacrifice, the higher the power. Now they must choose who are the three that need to sacrifice part of their bodies to gain power and who will wield the holy sword.