The story begins in the midst of a desperate battle for the Kingdom of Reinveldt. The capital city, a beautiful white castle by a lake, is under siege by a coalition of forces led by the Gurenshire Kingdom. Despite the valiant efforts of the kingdom's elite silver knights, they are overwhelmed by the sheer numbers of the enemy. As the castle gates are breached, the remaining male citizens and soldiers charge into the fray, buying time for the women and children to evacuate.
In the heart of the castle, the young knight Chris is on the verge of collapse. He is urged by his comrades to escape, but he refuses to give up. A boy named Jin, known as the Hero, uses his dark flame magic, "Dark Inferno: Black Flame Hell," to clear a path through the enemy soldiers. However, their numbers are too great. The knights, accepting their fate, make a final charge to buy Jin time to escape with Princess Chris. Jin, using his skills and remaining magic, rushes to find her. He finds Chris by a magic circle in a room that resembles a shrine.
Chris, surprisingly calm, reveals that she orchestrated Jin's return to his original world. She explains that they summoned him and that his efforts for them were deeply appreciated. Despite Jin's protests and desire to stay with his newfound family, Chris activates the magic circle, sending him back. As Jin is enveloped in light, he sees a tear fall from Chris's eye.
Jin awakens back in his classroom in Japan, a year having passed in his original world. He is a high school student, and the memories of his time as a hero feel like a distant dream, only confirmed by a ring he now wears. He attends a fan event for Sayama Rena, a popular voice actress he admires for her inspiring portrayal of a character who overcame helplessness. During the handshake event, a magic circle appears beneath Jin and Rena, and they are transported to another world.
They find themselves in a dimly lit underground room, surrounded by knights and a silver-haired girl named Lunaria Gurenshire, who introduces herself as the First Princess of the Gurenshire Empire. She explains that they have been summoned as Heroes to their world, which is different from their own. Jin, recognizing the world from his past experience, becomes protective of Rena. He discovers that he is now a slave, bound to Rena, a fact that surprises Lunaria and her knights, who seem to have expected a different outcome.
Rena, initially bewildered by the concept of magic and stats, begins to understand the world through Lunaria's explanations and her own familiarity with games. Jin, using his "Appraisal Magic Eye," discovers his own stats, which are significantly higher than expected, and his new status as Rena's slave. He also learns that Rena has been recognized as a Hero, indicated by a unique symbol in her occupation field.
The next morning, Jin and Rena are given luxurious accommodations. They discuss their situation, with Jin hypothesizing that the Empire intended to summon Rena as a slave, and he was inadvertently bound to her due to proximity. Lunaria expresses regret for their predicament and reveals that the Empire needs Heroes to combat monsters, as their military is engaged in war. She assures them they will not be forced into combat but will be trained.
Later, a slave merchant named Sandel arrives to formalize Jin's slave contract with Rena. Despite Jin's initial reluctance to break the contract, Lunaria insists, explaining that it is necessary for him to be recognized as a Hero and to ensure his safety. Jin accepts, and they are fitted with slave collars, which administer electric shocks for disobedience or attempts to harm their master.
The following night, Jin discovers he can manifest "Black Flame" and create a sword of pure black flame, a skill he honed in his previous life. He also learns that his "Item Ring," an artifact from his past life, can store vast quantities of items. Meanwhile, Rena's room is attacked by Gawain, Lunaria's arrogant older brother, who intends to assault her. Jin intervenes, defeating Gawain and the guards. Realizing they can no longer remain in the Empire, Jin and Rena decide to escape, aided by the knowledge that Jin can now use his powers and the Item Ring.
