The strongest agent, codenamed "Jupiter", was once the unstoppable protector of the ruined parallel world known as "B-Side." After 15 years of relentless revenge for his sister, he finally completes his mission and retires. Now, at 33 years old, he hides his superhuman abilities and starts a peaceful second life in the normal world, "A-Side." But his quiet life doesn’t last long—when a mysterious girl appears, his new life turns into a series of trials! Under a new name and in a new family, an unusual "under-one-roof" story begins!