During his summer vacation, 19-year-old Yami Yoshio is helping his family in the fields when he is suddenly transported to another world. He soon realizes that this is not a dream: dragons fly across the sky and strange creatures roam the land. In his pocket, he finds a letter from **Alice**, the single person who summoned him to this world. Alice asks Yoshio to save Nuroha Village from a troll attack. If he succeeds, he will receive special powers and may be able to return home.
On his way to Nuroha, Yoshio saves a young woman named Karen after her carriage crashes. Karen takes him to the village, where he meets her younger sister, Yoko. That night, trolls attack Nuroha. Yoshio tries to distract the attackers so that Karen and Yoko can escape, but the weapon he carries from his own world is useless against a powerful silver-class troll soldier. The troll explains that strength in this world is measured through a class system and that a low-ranked human cannot defeat him. Yoshio is killed by a blow to the head.
After his death, Yoshio awakens in a dark place and encounters a mysterious horned creature. The creature repeatedly asks him whether he wants to become stronger. Yoshio does not ask for power to save himself; he only wants to protect Karen and Yoko. Eventually, he accepts the offer. A white and dark substance rebuilds his body, and Yoshio is resurrected with an unknown power inside him. He defeats the troll soldiers and completes Alice's first test.
Yoshio, Karen, and Yoko then travel to the empire to meet Alice. In the city, they purchase new equipment, including a mysterious cursed sword that seems to call out to Yoshio. Alice explains the history of the world. Long ago, the king used forbidden demonic magic, and the gods punished the world by opening a dimensional gate through which monsters invaded. After the catastrophe, people were given a class system. Over time, the system became a tool for powerful individuals to control and oppress the weak.
The strongest known person is Tsuyoshi, a three-star God-class ruler who conquered the seven realms and forced them to submit to his order. Alice wants Yoshio to defeat Tsuyoshi and free the world from this oppressive system. She explains that the classes range from Class Zero and Bronze to Silver, Gold, Platinum, Diamond, Legendary, Immortal, and God, with a mysterious class beyond them all. A person must obtain three stars to advance to the next class.
Alice takes Yoshio to Fleep, a powerful Diamond-class master, so that he can begin training. Fleep introduces him to Mihoyi, a supernatural energy system. Basic Mihoyi can be used through weapons and sharp tools, while Positive Mihoyi strengthens the body and creates a powerful pressure. Negative Mihoyi is far more mysterious and dangerous. Fleep tells them that he once fought an enemy who used Negative Mihoyi to create terrifying attacks that caused pain and fear without relying on ordinary physical pressure.
Fleep tests Yoshio through a brutal physical battle. Yoshio is struck repeatedly, thrown into trees and rocks, and pushed to the edge of death. Even when he is bleeding and barely able to stand, he refuses to give up because he wants to return home and grow stronger. When ordinary combat proves insufficient, he turns to the cursed sword. The sword recognizes the strange monster hidden inside Yoshio and accepts him as its master after Yoshio endures its enormous weight. Yoshio forms a blood bond with the sword and learns to summon it from a distance. He manages to break Fleep's sealing technique and land a counterattack, but he collapses from blood loss. Impressed by his determination, Fleep accepts him as a student.
Fleep carries the wounded Yoshio to his old friend William Rosercrush, known as Will, a frightening-looking doctor. Will attempts to examine Yoshio's heart and sees enormous mysterious eyes inside him. The eyes repeatedly tell Will to die, causing blood to pour from his ears and eyes. Yoshio's wounds then begin healing on their own, convincing Will that the boy is not an ordinary Bronze-class human. After Yoshio wakes a week later, Will gives him his first mission: he must retrieve a rare Inari plant from Kyojin Mountain. Yoshio accepts the task and begins his journey through Buseon Forest.
Buseon Forest initially looks like a beautiful paradise, but it soon transforms into a nightmare filled with black mist, screams, blood, and skeletons. Yoshio discovers a mysterious door and passes through it, returning temporarily to his own world. After spending roughly three months in the other world, he reaches his family's home. His parents welcome him with tears and affection, but when Yoshio falls asleep, he hears a voice warning him that he does not belong there.
He wakes and discovers that his family is an illusion created by a demon. The food on the table is made from human remains, and an army of strange creatures is approaching from outside. The illusion tries to use Yoshio's longing for his parents against him. Realizing that he must destroy it, Yoshio cries and apologizes to his false family before killing them with a table knife. The doorway transforms into a demon, which cannot understand how a low-ranked human defeated it. Yoshio uses his cursed sword to kill the demon and rises to Bronze Class Three.
After leaving the forest, Yoshio reaches Chain Valley Village. He notices that every person in the village looks hideously disfigured, while an old woman is the only person who still appears normal. She explains that three years earlier, a demon named Osirus cursed the village. Osirus had once been beautiful, but after people rejected and envied him, he became a demon and stole the villagers' appearances. Every three months, he returns to control the villagers and sometimes takes their children.
Osirus is due to return in three days. Yoshio helps the old woman prepare a trap with the villagers. When Osirus arrives, the trap injures him, but he quickly recovers using Mihoyi and uses his power to control the villagers. Yoshio refuses to harm them and instead knocks them unconscious. He then fights Osirus alone. Although Osirus is stronger, Yoshio remembers Fleep's advice and refuses to retreat. He surrounds his sword with flames and manages to wound the demon.
Desperate, Osirus opens a gigantic rift leading to hell and attempts to drag the entire village into it. However, Yoshio's plan is already working. The old woman prepared a poison, and Yoshio placed it inside the first trap. Because Osirus is using his strongest attack, the poison spreads through his body more rapidly. The demon weakens and falls into the hellish rift created by his own power. Before disappearing, Osirus admits that he only wanted friends. With his death, the curse is lifted. The villagers regain their original faces, the mist disappears, and the village celebrates its freedom. The old woman gives Yoshio a valuable necklace as a keepsake, and he continues toward the Empire of Giants.
On the road, Yoshio witnesses a gigantic serpent fighting a massive giant. After a violent battle, the giant kills the serpent and continues on his way. Yoshio later encounters a young girl named Kaen, who helps him climb into the Empire of Giants with the aid of her companion, the sky demon Izanagi Gurea. Gurea has been Kaen's closest friend since childhood and can leap through the sky without wings.
Inside the giant city, Kaen becomes excited when she sees a beautiful magical peony. Yoshio buys it for her, and the gesture begins to change their relationship. Soon, the arrival of Whirlpool, an Immortal-class wandering marauder, throws the entire city into fear. Everyone is ordered to kneel and avoid looking at him. When a villager speaks disrespectfully, Whirlpool kills him instantly with a fishing hook. Yoshio is also injured when Whirlpool notices him, but Gurea heals the wound.
Yoshio and Kaen stay at an inn where humans and non-human beings live together. They can afford only one room, and Kaen becomes cold during the night. She moves close to Yoshio, and he embraces her to keep her warm. The next morning, Kaen becomes embarrassed and accuses him of being a pervert, even demanding that he take responsibility and marry her. Later, they go fishing at the Giants' Lake and cook their catch together.
At sunset, Kaen tells Yoshio about her past. Her parents abandoned her when she was a baby, and Gurea's family raised her. When she was twelve, she met a human boy and believed his promise that he would stay with her forever. Years later, that boy returned and massacred Gurea's family. Kaen was devastated and swore to take revenge. Yoshio tells her that he will help her. Kaen kisses his cheek, and the two exchange small keepsakes as a promise to remain connected.
To find the Inari plant, Yoshio and Kaen board the Raijin, a special railway powered by Tsuyoshi's strength. Monsters cannot attack the train. After the train passes the final station used by ordinary humans, it enters a region covered in thick mist. The conductor reveals that the train's staff are not real humans but energy beings created by Raijin's power.
The mist hides the Azıktı, road spirits from Turkish mythology that prey on travelers. They imitate the voices and appearances of the people their victims miss most. A seemingly harmless old man named Sato joins Yoshio and Kaen. He soon reveals himself to be one of the spirits. Sato takes the form and voice of Kaen's mother, while Yoshio hears the voice of his father. Yoshio briefly hesitates, but he recognizes the illusion because of his earlier experience in Buseon Forest and manages to wake Kaen.
Sato summons countless spirits and attacks them inside the train. The spirits devour passengers, tear through the carriages, and surround Yoshio and Kaen. The pair climbs onto the roof of Raijin and fights while the train approaches the Mikaboshi Tunnel. Sato creates increasingly powerful spirit attacks, including a horde that consumes other spirits and a massive city of spirits that attempts to devour Yoshio.
Kaen transfers some of her Mihoyi to Yoshio so that he can keep fighting. The spirits seize Yoshio and begin eating his body, making Kaen believe that he has died. However, the spirits cannot completely consume him. Yoshio realizes that his origin in another world has given him resistance to their attacks. He emerges from the mass of spirits and confronts Sato. Since Sato's strongest ability is ineffective against him, Yoshio advances with his cursed sword and defeats him with a single decisive strike.
Sato's body dissolves, the remaining road spirits flee, and Raijin finally emerges from the Mikaboshi Tunnel into a sunlit desert region. Yoshio and Kaen survive the journey, but their mission is not over. Yoshio still has to find the Inari plant, understand the mysterious power within his body, help Kaen confront the person responsible for her tragedy, and eventually challenge Tsuyoshi's oppressive rule. What began as Yoshio's attempt to return home has become a much greater journey in which he must grow stronger, protect the people he has come to care about, and uncover the true secrets of this dangerous world.