The elite assassin of the Demon King's army, known as "The Stealthy Viper," is given a dangerous mission: infiltrate the Hero’s party and earn their trust. To accomplish his goal, he takes his deception to the next level—even going so far as to defeat his own allies from the Demon King’s army. After all, "to fool your enemies, you must first deceive your friends." However, his talent doesn’t go unnoticed. The female hero, Nanami, who has never known a normal life, begins to trust him deeply… maybe a little too much. Instead of just an ally, she starts seeing him as a romantic interest!? A thrilling and unconventional isekai fantasy featuring an assassin who knows no limits and a hero who falls for him against all odds!