For undercover agents, carrying out their mission sometimes means pointing their guns even at their comrades. Among them is Rumie, codenamed Canary, whose kindness proves to be her greatest weakness. Her overly compassionate nature often leads to fatal hesitation in critical moments. Guiding her through this darkness is Kabura, her senior investigator and secret lover. However, having reached his limit with Canary's soft-heartedness, Kabura announces he will terminate her from the force. When Canary pleads for one final chance, Kabura assigns her a new mission—promising to marry her upon its completion. Behind this promise lies a hidden trial, designed to reveal the true nature buried within her.