The story begins by establishing the world's system of "Titles," which are earned through experience and define a person's life path. However, the ultimate Title, "Hero," is bestowed by the gods and grants special power, making Heroes humanity's hope against the Demon race.
The protagonist, Ast, recounts his past. At sixteen, he was recognized as a Hero by a priest and taken to the Temple of the War God for training. He was once a simple village boy with dreams of becoming a warrior. His Hero designation opened a path to fulfilling those dreams, which he eagerly embraced. At fifteen, he entered the fifteen-floor "Trial Grounds of the War God," a dangerous underground labyrinth filled with traps, monsters, and treasures, designed to train warriors and adventurers.
There, Ast met two other Heroes: Eurich Quorval and Sevarna Kalentz, both nobles from neighboring countries. Their instructor revealed that the three would eventually gather in Granfort to fight demons, and they were to train together. Ast, a commoner, was treated coldly by Eurich and Sevarna, who seemed offended by his equal Hero status.
During their training, Ast focused on swordsmanship and basic healing magic, while the others did the same. However, their party lacked essential roles like a rogue for lockpicking and trap detection, or a priest for healing, or a sorcerer for offensive magic. When searching for party members in the nearby town, Ast was ignored, while Eurich and Sevarna quickly found companions. This disparity continued in the labyrinth; Ast received delayed or no support, even when injured, while Sevarna was prioritized. He realized they intended to kill him, framing it as a death in the labyrinth to avoid scandal. This revelation plunged Ast into despair, but his pride and anger fueled his resolve to survive and grow stronger alone. He learned to heal himself, studied magic books, and observed rogues to steal their techniques.
After a year of solo training within the party, Ast and his companions reached the fifteenth and final floor of the labyrinth. The night before, Eurich and Sevarna were celebrated by adventurers, while Ast prepared alone. The fifteenth floor presented a significantly greater challenge, forcing the three Heroes to cooperate for survival. Ast felt a flicker of hope for camaraderie, but this was short-lived. When Sevarna fell into a trap, Ast instinctively saved her, falling into the pit himself. He managed to cling to the edge, but Eurich pulled Sevarna away, abandoning Ast. Realizing their betrayal, Ast laughed at his own naivete and fell into the abyss, hearing a voice welcome him to "Hell Route."
The labyrinth continued beyond the fifteenth floor, becoming a true trial. Ast survived by hunting monsters for food, learning to rest in short bursts, and utilizing the divine nature of the labyrinth's creatures and items. He discovered that clues to conquering each new floor were hidden within the current one, allowing him to acquire items like a water-breathing amulet and night-vision goggles before facing corresponding challenges. He also gained access to an "Infinite Storage" key. After an indeterminate amount of time, during which he lost his sense of time and his desire to return to the surface, Ast reached the hundredth floor, the labyrinth's true end.
There, he encountered a Dark Elf, a type of demon. They were forced to fight together against a colossal Iron Giant, the final trial. After a grueling battle, they defeated the giant by coordinating their attacks, exploiting its seven-attribute barrier. The giant dissolved into a divine water that healed them and revealed a long staircase leading upwards. Ast and the Dark Elf, sharing a profound connection forged in their shared ordeal, embraced before ascending.
Upon returning to the surface, Ast found himself in an unfamiliar forest. He was disoriented, unsure of his purpose, and his past as a Hero felt distant. He hid his powerful gear in the Infinite Storage and, clad in rags, headed towards a nearby settlement. Overwhelmed by hunger, he collapsed at the doorstep of a young woman named Stella. Stella, a kind girl of about fifteen, offered him food and shelter, believing his story of being robbed and losing his memory. Ast, now appearing to be around twenty, adopted the name Lunark, Stella's deceased brother's name, to avoid drawing attention.
While helping Stella with chores, Lunark noticed the villagers' wary glances and Stella's evasiveness about the nearby forest. He discovered the village was plagued by undead, and Stella's brother, the original Lunark, had died investigating them. The undead were drawn by people venturing out at night, and the village was suffering losses. Lunark, armed with his brother's sword and his own combat skills, decided to confront the undead. He learned the village was also threatened by a vampire, who had killed Stella's brother. Lunark, recognizing the vampire as a lower-tier monster, confronted her in her lair. He revealed his immense power, forcing the vampire to beg for mercy. He then took her and Stella and Crezon, the village lord who was revealed to be Stella's half-brother, to a place where they could become vampires, deeming them unworthy of living as humans. He then left the village, leaving the now-vampire Stella and Crezon to their fate.
Continuing his journey, Lunark encountered a holy maiden named Tetefi, an albino priestess. He provided her with food and water, but she fled in panic when she recognized him as the one who had helped her escape knights from the Quorval family, who were trying to force her into a ritual. Lunark, recognizing the Quorval crest, decided to pursue them, his past betrayal by Eurich fueling his decision.
