The heir to a centuries-old family of assassins known as Poison Masters, Hikaru Gero has spent his life in the underworld with no experience in romance or marriage. When the head of the Gero family decides the bloodline must continue, Hikarus sister is ordered to bear an heir, a fate he cannot accept. During an assassination mission, Hikaru is sent to kill Mei Kinosaki, a brilliant and charismatic marriage swindler who cons people for money to fund her younger brothers medical treatment. Instead of completing his task, Hikaru makes an unexpected proposal: he asks Mei to help him find a wife.

Mei accepts, becoming Hikarus unconventional marriage consultant. Their strategy to find Hikaru a partner involves taking on missions to help people in need, believing that acts of heroism will make him more appealing. Hikaru uses his mastery of toxins to enhance his physical abilities and subdue enemies, while Mei uses skills of seduction and psychological manipulation to gather information and navigate social situations. Together, this unlikely duo faces a series of dangerous adversaries who possess unique abilities passed down through generations, including the Water User, the Sound User, and members of other powerful families like the Beast Users. Early missions include rescuing Kyoko Himekawa, a thief who steals back art from corrupt collectors, and serving as a bodyguard for Shiori Ureshino, a corporate heiress.

As Hikarus marriage activities continue, he encounters a world of specialists, assassins from elite families with genetically honed skills, each belonging to a specific clan such as the Poison Users, Water Users, or Sound Users. He clashes with rivals like the branch family member Teruaki, who also targets Mei. The series balances intense action sequences with comedic moments, often highlighting Hikarus extreme social awkwardness around women, which contrasts sharply with his deadly competence in combat. Throughout his journey, Hikaru forms genuine connections with potential romantic interests, including the shark-obsessed thief Kyoko, while Mei remains a constant presence as his advisor and friend, revealing more of their own past and motivations as the story progresses. The narrative explores themes of duty versus personal freedom, finding connection in a violent world, and the many different forms love can take.