Kakyu, a former Japanese who was reincarnated as a lower-class devil in hell, has acquired unparalleled strength over the course of 2,000 years. However, he is unexpectedly banished to an "other world" and freed from the harsh hell. Kakyu was enjoying the slow life he had longed for, but the village he was taking care of was attacked, and although he rushed there, the village was wiped out. Picking up the only surviving baby, Ars, he decides to repay his debt by raising Ars to be the happiest human in the world...!?