Tenka Kumo and his younger brothers Soramaru and Chutaro run a shrine on the shore of Lake Biwa and ferry convicts to Gokumonjo, a prison built on an island in the middle of the water. Swords have been banned in public under the new Meiji government and crime has climbed, so the brothers escort the worst offenders across the lake themselves. Beneath the routine lies an older threat: the serpent-god Orochi, said to revive every three centuries, and the human vessel meant to hold it.