The protagonist, a regular office worker, suddenly finds himself in a dungeon, inexplicably equipped with a sword and shield. He quickly realizes this resembles the RPG "Brave & Blade," a game he was obsessed with in his youth. He recalls dying in a traffic accident while trying to save a schoolgirl, and a voice saying his wish was granted. He encounters a goblin, initially terrified, but his body instinctively moves, and he defeats it with surprising skill. The goblin dissolves into particles that are absorbed into his left hand, mirroring the game's experience point system.
Looking at his reflection in the sword, he sees not his own face, but that of "Rex Thoren," a popular but ultimately minor character from "Brave & Blade." He deduces he has reincarnated into the game world as Rex. He laments being a "sub-character" with high initial stats but no future potential, unlike the protagonist or other popular characters. He recognizes the dungeon as the "Trial Cave," the starting area for the "City-Dweller Aspiring Adventurer" origin. He realizes with dread that this city, "Earth," is about to be attacked by monsters, a key event in the game.
He rushes to prevent the "protagonist" from triggering the demon seal in the cave, believing he can alter the game's events. However, he finds the seal remains unbroken, and the group of adventurers he encounters doesn't match the expected protagonist party. He deduces the protagonist must have chosen a different origin. He then encounters a group of novice adventurers: the hot-headed swordsman Rad, the calm mage Newk, the perceptive archer Prana, and the healer Mana. He helps them escape the cave, but they witness the city of Earth burning.
The protagonist, as Rex, realizes the city's destruction is unavoidable, regardless of the protagonist's origin. He faces a choice: flee to the fishing village of Umina, where he would die protecting the others, or flee to the town of Rixia, where the novice party would be ambushed and killed by gargoyles. He chooses a third option: to lead the group back into the "Trial Cave" through a hidden shortcut to "Winged Dragon's Promenade," a more dangerous area.
During their escape through the shortcut, Rex uses his game knowledge to gather valuable items, including "Mystic Brine" and "Phantom Powder of Restoration," which he uses to heal Mana's sister, Recilia, who was attacked by a gargoyle. He then faces a Level 60 Doom Demon, a powerful monster that appears when fleeing to Umina. Using his knowledge of game mechanics, including manual Arts activation and item buffs, Rex fights the demon. He manages to inflict significant damage, but the fight is a "defeat event" for Rex, and he is killed by the demon's powered-up form.
However, the "Phantom Powder of Restoration" activates, reviving him. He realizes he can use this item to survive events that would normally be fatal for him, as he is a sub-character and not the protagonist. He then witnesses the "Goddess's Oracle," a world-spanning event announcing the impending revival of the demon god "Raslufi" and the start of a new era of conflict.
The protagonist, now fully embracing his role as a survivor rather than a hero, decides to let the true protagonist handle the world-saving. He plans to use his game knowledge to live comfortably and make money. Meanwhile, Rad, inspired by Rex's fighting prowess, begins to experiment with manually activating Arts, believing he can reach Rex's level. The narrative then shifts to Rad's perspective, revealing his admiration for Rex and his determination to become a hero. He is visited by Rex, who, surprisingly, asks to borrow money, claiming he needs it for gambling. Rad, though initially suspicious and annoyed, agrees to lend him the money, intrigued by Rex's unpredictable nature and the potential for future adventures. The chapters conclude with Rad realizing the true potential of manual Arts activation and his resolve to catch up to Rex.
