In a devastated parallel world known as the B-side, the most powerful agent codenamed Jupiter served as an invincible guardian. After fifteen years of hunting down those responsible for his sister’s death, he finally achieves his vengeance and chooses to retire from his life as an agent. Now 33 years old, he conceals his superhuman abilities and crosses over to the peaceful A-side, the main reality, hoping to live a quiet second life in anonymity. However, his plans for tranquility are shattered with the sudden appearance of a girl named Makoto, who comes from the same parallel world and carries her own complicated circumstances. She becomes his unexpected housemate, and her presence forces the former agent to confront a new kind of challenge. Adding to the emotional complexity, Makoto is the same age as his late sister. Adopting a new name to hide his past as Agent Jupiter, he begins a strange life under one roof with this girl, trying to balance the ordinary existence he craves with the lingering shadows of his former identity as the legendary agent now calling himself Mokusei.