"Shōnen Onmyōji" is a manga series set in Heian-era Japan, blending historical elements with supernatural themes. The protagonist, Abe no Masahiro, is the grandson of the renowned onmyōji Abe no Seimei, a legendary figure in Japanese folklore known for his mastery of spiritual and mystical arts. Masahiro, however, lacks the spiritual sensitivity required to become a skilled onmyōji, leaving him overshadowed by his grandfather's legacy. Despite this, he aspires to follow in Seimei's footsteps and prove his worth.

The narrative begins with Masahiro encountering a mysterious spirit named Mokkun, who serves as his guide and protector. Mokkun reveals that Masahiro possesses latent spiritual abilities, which gradually awaken as the story progresses. Together, they embark on a journey to combat malevolent spirits and supernatural threats plaguing the capital. Along the way, Masahiro faces various challenges, including political intrigue, rivalries among onmyōji, and personal struggles to reconcile his insecurities with his growing powers.

Key characters include Abe no Seimei, who serves as a mentor figure despite his aloof demeanor, and Guren, a powerful shikigami with a complex relationship to Masahiro. The series also introduces a range of supporting characters, such as court nobles, fellow onmyōji, and spirits, each contributing to the intricate world-building. The plot intertwines episodic supernatural cases with an overarching storyline involving a conspiracy that threatens the balance between the human and spirit worlds.

As Masahiro matures, he gains a deeper understanding of his responsibilities as an onmyōji and the moral complexities of wielding spiritual power. The series explores themes of duty, legacy, and the coexistence of humans and spirits, all set against the richly detailed backdrop of Heian-era Japan. The narrative maintains a balance between action, mystery, and character development, making it a compelling exploration of the onmyōji tradition and its place in Japanese history and mythology.