High school student Akira Oda and his entire class are suddenly summoned to the fantasy kingdom of Retice. While his classmates are bestowed with legendary classes like Hero, Akira receives the unassuming role of Assassin. However, he quickly discovers that his status as an Assassin is extraordinarily high, with starting stats that even exceed those of the Hero. Suspicious of the king who summoned them, Akira uses his natural lack of presence and newfound abilities to investigate the royal court. He uncovers a dark conspiracy, but before he can act, he is framed for murder and driven into exile. Swearing vengeance against the king, Akira flees into the unexplored depths of the Great Labyrinth of Kantinen. Inside the labyrinth, he encounters Amelia Rosequartz, a skilled elven princess and Silver-rank adventurer, and Night, a black cat monster who becomes his companion.

The narrative follows Akira as he embraces the path of the Silent Assassin, focusing on stealth, reconnaissance, and precise strikes. His journey is marked by a central conflict: he possesses overwhelming power but struggles with the moral weight of taking a life, leading to prolonged internal debates about whether to assassinate his enemies. The story is driven by his quest to clear his name and exact revenge on the corrupt King Edmond and his allies, including the scheming Lord Gram. Akira and Amelia are eventually joined by Crow, a master blacksmith who was a former member of the previous Hero's party, and Lia Lagoon, a mysterious girl who knows the secret behind their summoning. The plot expands beyond personal vengeance as the group discovers that the Demon Lord may not be the true villain, with figures like the demon Aurum Tres acting under complex orders, forcing Akira to question who his real enemy is.

The setting begins in the Kingdom of Retice, a realm with a corrupt monarchy that manipulates summoned heroes for political gain. The story then moves through dangerous dungeons, the elven homeland where Akira must duel Amelia’s sister Kilika, and the Beastmen continent where the party confronts Lord Gram. Notable narrative arcs include the introduction of Akira’s overpowered but underutilized skills, his bonding with Night, the fracturing of his classmates into rival factions, and the quest to uncover the conspiracy behind the summoning ritual. Ultimately, the tale follows Akira’s evolution from a forgotten background character into a reluctant avenger, using his shadowy powers to navigate a world of political intrigue while forming deep bonds with his elven and beast companions.