"Koro Sensei Quest!" is a spin-off manga series that reimagines the characters and setting of "Assassination Classroom" in a fantasy RPG world. The story centers around Class 3-E of Kunugigaoka Academy, a group of students tasked with defeating the Demon King, Koro Sensei, a powerful and enigmatic octopus-like being. Unlike the original series, where Koro Sensei is their teacher, here he serves as the ultimate antagonist, ruling over the world with his overwhelming strength and magical abilities.

The students, each equipped with unique classes and abilities typical of RPG archetypes, embark on a quest to level up, gather resources, and strategize to take down Koro Sensei. The narrative blends humor, adventure, and lighthearted fantasy tropes, with the students facing various monsters, dungeons, and challenges as they progress. Key characters include Nagisa Shiota, a versatile and quick-thinking rogue; Karma Akabane, a powerful and rebellious warrior; and Kaede Kayano, a skilled mage with a mysterious past. Other classmates fill roles such as healers, archers, and knights, contributing to the party dynamics.

The series incorporates parody elements of classic RPG mechanics, such as leveling systems, item crafting, and boss battles, while maintaining the core personalities and relationships from the original "Assassination Classroom." Koro Sensei himself remains a comically overpowered figure, often toying with the students but also providing guidance and opportunities for growth. The story balances episodic adventures with an overarching goal, culminating in the students' efforts to confront Koro Sensei in a climactic battle. The tone remains light and comedic, with occasional moments of character development and camaraderie that reflect the original series' themes of teamwork and perseverance.