In South Korea, Gates open for unknown reasons. Researchers speculate they are passages to other worlds, but the underlying principles remain a mystery. Approximately one hour after a Gate opens, various monsters emerge. Hunters prevent damage by entering to defeat the boss or by sealing the passage from the outside.

Hunters are ranked from F to S, growing through experience, training, and skill acquisition. Rising even one rank is difficult, two is rare, and three is nearly impossible. There are five S-rank hunters in South Korea. Two belong to Helios, while Changcheon, Jeokwol, and Baekya each have one; these four are known as the major guilds. In an era where guilds hold significant power, the Association handles registration, screening, and Gate monitoring. The strongest independent hunters are A-rank, handling weak Gates personally, while the conquest of strong Gates is commissioned to the guilds.

Ten years ago, Guest was a rookie C-rank hunter in the mid-sized guild Blue Sphere. Expected to potentially grow by two ranks, Guest set out on a dungeon raid with four young comrades: Kang Taehyuk, Lee Seoha, Oh Jinwoo, and Moon Sojung. During the mission, a mysterious Gate opened inside the dungeon. Guest was sucked into the passage, and the Gate vanished. No one witnessed the death, but the remaining comrades could find neither a way back nor a way to rescue Guest.

Guest arrived in an otherworldly realm resembling ancient Eastern culture. Sects and masters vied for power, and people fought by manipulating Qi into forms like sword energy and palm blasts. While searching for a way home, Guest fought endlessly for survival. After ten years of unintended cultivation, Guest reached the Transcendent Realm and finally unified that world. At that moment, Guest returned to the original world. How unification and return are linked remains unknown. No one can go back to that world now; the time spent there remains only in memory and the techniques left in the body.

Upon returning, Guest's strength is at the S-rank level, but official records still list Guest as a C-rank missing person from ten years ago. The Qi-based combat style is foreign to modern hunters, and Guest is unfamiliar with the equipment and lifestyle that emerged during the decade away. The streets, the guilds, and the people have all changed.

Kang Taehyuk left Blue Sphere to become a famous S-rank in Helios. The drive to grow for a dead comrade eventually turned into an ambition for higher status. Oh Jinwoo became a top-tier A-rank independent hunter, remembering Guest as a long-lost friend. Moon Sojung lives as a C-rank in the Solip Guild, carrying the trauma of the failed raid. Lee Seoha stayed to protect Blue Sphere as the A-rank Vice Guild Master, tracking traces of Guest's survival for ten years without success.

However, Guest has not yet met any of the old comrades. Their current affiliations and lives are unknown. The story begins as Guest visits the guild from memory, Blue Sphere. Passing through the changed streets wearing a worn, old-model coat over otherworldly undergarments, Guest stands before the guild's door. It is yet unknown who behind that door will remember, or what their reaction will be.