Waking up cold and alone in the woods, a nameless man is surrounded by unfamiliar scenes. Without any memories, he is utterly lost until he stumbles upon Kuon—a young girl with animal ears and a tail. She saves him, leading him back to her town while protecting them both from "boro-gigiri"—giant centipede-like creatures—and a mysterious red slime. After they arrive, she names him Haku after a prominent historical figure from the area. He quickly realizes he is one of a kind—the only one without animal ears and a tail. The two journey to the capital of Yamato, the country in which Kuon lives, meeting more people and making new friends, all the while unknowingly delving deeper into the politics and inner conflicts of Yamato... Utawarerumono: Itsuwari no Kamen follows Haku as he slowly uncovers the secrets of this alien world. He works to solve the mysteries surrounding the red slimes, as well as to stop war from erupting among the nearby countries.

In her house, Mei Onigami discovers a strange round creature that only says "chimi." Her sisters, Mutsumi and Hazuki, agree to let the being stay with them until its owner comes to claim it. However, as soon as one of them utters the word "hell," more creatures show up until there are 12 in total. The sisters think nothing of this until the critters create a portal that summons an envoy from Hell. Jabberwock the Eighth, a horned and fanged man covered in fur, has come to unleash a living hell on earth under orders from his boss. However, when living in the human realm proves to be challenging, the Onigami sisters allow him to stay with them. They also take in his "Chimimoryo spirits"—the round demons that preceded his arrival. Called Jigoku-san by the Onigami sisters, this messenger from Hell has a lot of work cut out for him if he wants to take over the world. But before that, he will have to survive the everyday hells of human life.