Hikaru Gero is the heir to the Poison Clan, a centuries-old family of professional assassins who have perfected the art of killing using toxins. Because he has spent his entire life in the underworld and never experienced a normal life, Gero considers himself hopeless with women and has no interest in marriage. However, the clan elders deliver an ultimatum: unless Gero marries and produces an heir, his younger sister Akari will be forced to bear a child against her will. To protect her, Gero decides to find a wife. On his next assignment, rather than killing his target—a cross-dressing marriage swindler named Mei Kinosaki—Gero instead proposes marriage on the spot. Kinosaki refuses but, intrigued by the unusual request, agrees to become Gero's relationship coach and train him in the arts of dating and romance so he can find a woman he genuinely wants to marry.

The story unfolds in a world where the Poison Clan is one of the five great families of Masters, each commanding unique abilities. Gero operates as a hitman with a personal code, targeting only those he considers human scum. His early attempts at romance fail miserably due to his complete lack of social skills, but after he rescues Kinosaki from a kidnapper, his coach realizes that Gero's true appeal lies in his deadly competence. Kinosaki then reframes his assassin talents as heroic rescue opportunities, turning his missions into meet-cute scenarios with potential brides.

The first major arc involves Gero accepting a commission to rescue the thief Kyoko Himekawa, a vigilante who steals art to return it to its rightful owners. During the rescue, Gero battles the Water Master Ushio in a fierce fight. Afterward, he and Himekawa go on a date, where he awkwardly follows Kinosaki's coaching; Himekawa responds warmly, and Gero asks to begin their relationship as friends. In a subsequent mission, Gero goes undercover as the bodyguard Daigo Oshima for Shiori Ureshino, the socially anxious heiress of a party-supply company. During a university camp, a Sound Master attacks, targeting Ureshino, and Gero must protect her while navigating both the combat and his ongoing quest for a suitable partner. Later storylines introduce a hitman hunter stalking the underworld, the Beast Master of another great family who poses a deadly threat, and a rival from a branch of the Gero family who targets Kinosaki. Throughout these battles, Gero refines his skills, confronts other Master families, and continues his awkward but determined search for the ultimate marriage—all while balancing his assassin duties and the growing complications of his partnership with Kinosaki.