In a near future Japan devastated by a chronic and worsening birth rate decline, the nation has implemented an unprecedented state policy to combat its demographic crisis. The yearly death toll has surpassed three million, and with the country facing potential extinction, the duty of procreation has been added to the list of mandatory citizen obligations alongside taxation, education, and labor. Under this emergency measure, girls who have completed basic training after turning fourteen are designated as Shoushika Tantou Shoujo, or declining birthrate officers, and are distributed to live with single men. This is the world that thirty-four year old aspiring novelist Tsubaki Hideki inhabits. Disillusioned with the state of society and his own life, he has become a reclusive shut-in, living off his parents assets. The government has also imposed a penalty of one billion yen for any man who has not fathered a child by the age of thirty-five, a fate that is rapidly approaching for Hideki.

The story begins with the sudden arrival of a sixteen-year-old girl named Suika at Hideki home. With a formal and earnest demeanor, she declares, Please let me make a baby with you. Suika is a highly responsible and serious student who graduated from the Etajima National Rescue Brides School, an institution designed to train these young girls for their role. As Hideki despairs over his circumstances, Suika convinces him to cooperate by reframing their mission not just as an obligation, but as a chance to change the future of Japan for the children yet to be born. Despite her strong sense of duty, Suika is also extremely shy and, due to state regulations and her sheltered upbringing, completely innocent about romance and physical intimacy. The manga primarily follows this mismatched pair their awkward, often comedic, and increasingly complex cohabitation as they navigate the pressure to fulfill their national duty.

Hideki himself struggles with deep-seated feelings of worthlessness and a creative spirit stifled by government censorship. The state only permits the writing of propaganda light novels focused on romance and pro-natalist themes, a restriction that fills him with despair. An early narrative arc focuses on Hideki and Suika inability to progress physically due to their respective anxieties and her overwhelming shyness. This stalemate is disrupted by the arrival of Ichigo, Suika younger and even more assertive classmate from the brides school. Ichigo forces the pair to confront their feelings, directly asking Hideki to teach them about love, as Suika has only ever viewed the act of creating a family as a mission to save Japan, devoid of personal emotion. This creates a foundational conflict where Hideki must decide if he is simply complying with a law or forming a genuine relationship.

As the volumes progress, the series expands its cast and setting. Another declining birthrate officer named Momo appears, who aggressively pursues Hideki and creates a rival dynamic, pushing Suika to experience jealousy and other new emotions for the first time. In a later arc, Hideki receives state recognition for writing a risqué novel, leading to a new commission and forcing him and Suika to engage in more public and provocative research. On a trip to a decaying, abandoned shopping mall to find ideas for his stories, the pair face external dangers from the crumbling infrastructure and encounter the Grand Master of the Emperor, a higher authority figure who dramatically shifts the stakes of their mission. Through the volumes, the story continues to blend its premise of extreme state-mandated relationships with a narrative about two socially awkward individuals gradually learning to understand themselves and each other.