A veteran gamer known only by his player name Zephyrus has spent over 8,000 hours mastering the legendary dungeon-crawl RPG Dungeon Shūkatsu no Susume, often shortened to Dan-Katsu. After suddenly losing all of his save data, the shock causes his death. However, when he awakens, he finds himself reincarnated inside the very world of the game he knew so well. Rather than fear, he feels overwhelming joy. Immediately granted the classic and most powerful job class, the Hero, Zephyrus decides to start a new game in this unconquered reality.

The setting is the world of Dan-Katsu itself, a massive fantasy system featuring over 1,021 playable classes and more than 400 possible endings, where the main story primarily unfolds at Labyrinth Academy. Zephyrus enrolls at this academy, armed with an encyclopedic knowledge of every game mechanic, secret, and dungeon layout. He discovers that what was considered a for-fun class or a challenge back in his original world is perceived very differently here, and that many of the most difficult dungeons have never been successfully conquered.

Zephyrus quickly uses his expertise to gather a party of powerful allies. He guides his reserved childhood friend toward becoming an Alchemist and transforms a selfish princess into a Saint. Together, this unique team of classmates, equipped with classes that Zephyrus knows are optimal, sets out to conquer the dungeons and explore every corner of the game world as if playing a brand new adventure for the first time. His journey is defined by using his past life's gaming experience to break the unreached limits of this world, turning what should be a dangerous isekai into an exciting, strategic, and highly enjoyable new playthrough.