"Disgaea 3: School of Devils" is a manga adaptation of the tactical role-playing game of the same name, set in the Netherworld, a realm inhabited by demons. The story centers on Mao, the son of the Overlord of the Netherworld, who attends the Evil Academy, a prestigious school for demons. Despite his lineage, Mao is an underachieving student who despises honor students, particularly his father, whom he views as a hypocrite for preaching evil while acting benevolently. Mao's primary goal is to surpass his father and become the ultimate embodiment of evil.

The narrative unfolds as Mao discovers a legendary hero, Almaz, who has been transformed into a Prinny, a penguin-like creature often used as a disposable servant in the Netherworld. Almaz seeks to rescue his fiancée, Sapphire, who has been kidnapped by Mao's father. Mao, seeing an opportunity to exploit Almaz's heroism for his own gain, forms an uneasy alliance with him. Together, they navigate the chaotic and often absurd hierarchy of the Evil Academy, encountering a diverse cast of characters, including Mao's loyal vassal, Raspberyl, and the enigmatic teacher, Mr. Champloo.

As the story progresses, Mao's motivations and understanding of evil are challenged. His journey reveals deeper layers of the Netherworld's politics, including the existence of a secret organization manipulating events behind the scenes. The plot intertwines themes of rebellion, self-discovery, and the blurred lines between good and evil, as Mao and his companions confront both external adversaries and their own personal flaws.

The manga retains the humor and over-the-top antics characteristic of the "Disgaea" series, blending comedic moments with strategic battles and character-driven drama. Mao's growth from a self-centered underachiever to a more nuanced figure forms the core of the narrative, while the supporting cast adds depth and variety to the story. The Evil Academy serves as a dynamic backdrop, with its absurd rules and eccentric faculty providing both challenges and opportunities for the protagonists. The manga concludes with Mao and his allies facing off against the true orchestrators of the conflict, culminating in a resolution that redefines Mao's understanding of power, evil, and his place in the Netherworld.