High school student Akira Oda is summoned to a fantasy world along with his entire class. While his classmates are granted powerful cheat abilities befitting heroes, Akira receives the unassuming class of Assassin, a role that seems to match his naturally unremarkable presence. Despite this, his status parameters prove to be suspiciously high, even exceeding those of the Hero class.

Akira quickly grows suspicious of King Edmond of the kingdom of Retice, who orchestrated the summoning. He begins secretly investigating the king's motives, using his stealth abilities to uncover an evil scheme. However, before he can act, Akira is falsely framed for a crime and forced to flee the kingdom, swearing vengeance against the king.

Escaping into the unexplored depths of the Great Labyrinth of Kantinen, Akira encounters Amelia Rosequartz, a skilled elven princess and Silver-rank adventurer. Normally quiet and aloof, Amelia begins to open up after Akira saves her life. Together, they journey through the labyrinth, where they defeat the boss of the 80th floor—a black cat monster named Night. Night takes a liking to Akira, forming a bond with him and becoming a companion.

After their adventures in the labyrinth, Akira accompanies Amelia to her homeland. There, he becomes entangled in elven politics, including a conflict with Amelia's younger sister Kilika, whose memories have been tampered with by their father. Akira helps resolve the family's troubled history, putting an old elven grudge to rest.

The journey continues to the beastman continent, where Akira seeks a blacksmith worthy of reforging his sword Yato-no-Kami, a weapon once wielded by the first Hero. He meets Crow, a skilled blacksmith who was once a member of the previous Hero's party. Crow's tragic past and goals begin to align with Akira's, as both realize there is a mastermind behind the kingdom's schemes.

Along the way, Akira is joined by other allies, including Lia Lagoon, a mysterious girl who knows the secret behind the summoning of Akira and his classmates. The group faces threats from demons, including the powerful Aurum Tres, who attacks a city and kidnaps Amelia. Throughout these trials, Akira struggles with his own morality—he has never killed anyone and is reluctant to do so, even as his assassin abilities and his desire for vengeance push him toward darker choices.

As Akira and his companions uncover more about the king's corruption and the true nature of their summoning, he must balance his goal of returning home to his ill mother and younger sister with the bonds he has formed in the fantasy world, particularly his growing feelings for Amelia.