In the crumbling Kio Empire, five-year-old Ageha is the neglected and overweight eldest son of a powerful noble count, derisively called a white pig. Ageha carries a secret advantage: he was born with the complete memories of his past life as a Japanese person, which gives him a mature perspective and skills like cooking, sewing, and crafting.

His already difficult life takes a sharp turn when he meets his infant half-brother, Regulus. Ageha has a prophetic vision revealing that Regulus is destined to kill him in the future as an adult over their noble inheritance. Instead of fearing or resenting the child, Ageha decides to alter his fate through compassion. He resolves to raise Regulus with love and proper care, hoping to prevent the dark future he has foreseen.

The story follows the two brothers as they navigate the hostile environment of the Kikunoi (or Bernstein) household, a family rife with corruption and political scheming. Using the domestic skills from his previous life, Ageha creates handmade toys, prepares nutritious meals like chawanmushi, and teaches his brother moral values. The siblings are aided by a cast of unusual allies, including the stern head maid Adelheid Rottenmeier, an elven teacher named Aleksei Romanov who instructs them in magic and world knowledge, and even whimsical gods who take an interest in their efforts.

As Ageha uses his past-life wisdom to slowly reform their territory and bring prosperity to the people, the narrative expands from a focused sibling survival story into a larger fantasy about governance and nation-building, where the actions of two young boys begin to change the entire country.