"How NOT to Summon a Demon Lord" centers on Takuma Sakamoto, a socially awkward gamer who is transported into the world of his favorite MMORPG, Cross Reverie, as his in-game character, the powerful Demon Lord Diablo. Upon arrival, he is summoned by two girls, Rem Galeu, a Pantherian (a beast-human hybrid), and Shera L. Greenwood, an elf princess. Both attempt to bind him with a magical contract, but due to a magical mishap, the spell backfires, and the binding instead enslaves them to Diablo.

Rem and Shera summon Diablo to help them with their respective struggles. Rem seeks to free her people from oppression, while Shera aims to escape an arranged marriage and reclaim her kingdom. Despite his overwhelming power, Diablo struggles to adapt to his new reality, often masking his social anxiety by acting in line with his in-game persona as a confident and domineering Demon Lord. This duality creates a blend of comedic and dramatic moments as he navigates his relationships with Rem, Shera, and the world around him.

The narrative explores themes of power, responsibility, and identity as Diablo grapples with his role in this new world. His journey involves forming alliances, battling formidable foes, and uncovering the mysteries of Cross Reverie. Along the way, he encounters a diverse cast of characters, including other adventurers, magical beings, and antagonists who challenge his abilities and worldview. The story balances action, humor, and character development, with a focus on the evolving dynamics between Diablo, Rem, and Shera as they confront their personal and shared challenges.

The series incorporates elements of fantasy, adventure, and harem genres, with a mix of lighthearted moments and darker, more serious undertones. The world-building draws heavily from RPG mechanics, with references to levels, skills, and item systems, creating a familiar yet immersive setting for fans of the genre. The interactions between the characters drive much of the narrative, with Diablo’s growth as a leader and individual serving as a central thread throughout the story.