In the small kingdom of Hazenveria, with a population of approximately 50,000, a young man named Slein, raised as an ordinary citizen, suddenly discovers he is the illegitimate son of King Frederik Hazenveria. After a tragic accident wipes out the rest of the royal family, Slein becomes the sole heir and the next king. Unsure of himself at first, Slein gradually grows into his role, developing his talents and abilities. He navigates the complex political landscape of small nations and great powers, striving to protect and enrich his homeland. The story starts slowly, focusing on internal affairs before gradually shifting towards military conflicts.