After staying up late playing an adult video game, Itsuki Kusaka wakes up to find himself in a completely unfamiliar world. He has been summoned to Marona, the Flower Capital, a city where men are strictly forbidden. This holy place is ruled by the Four-Flower Goddess, who is nearing the end of her thousand-year reign and must pass her divinity to a successor. The candidates for the next goddess are four innocent nuns: Lily, Talia, Rose, and Lupina. To qualify, they must mature both as women and as potential mothers, but they have been raised in complete ignorance of sexual matters, believing such things to be sinful. The goddess gives Itsuki an unusual mission: he must teach the four candidates about sex through hands-on practical instruction. Only after this task is complete will he be allowed to return to his own world. Itsuki is given a ring called the Stamen Scripture, which glows and reveals new challenges as the candidates are ready to progress to the next stage of their education.

Itsuki is an ordinary salaryman who is initially bewildered by his situation. He approaches the task with a sense of responsibility and tries to treat the sisters with sincerity, though he struggles with the ethics of his role and the feelings that begin to develop between them. Lily is the most serious and responsible of the sisters, and she becomes the first to receive guidance, progressing from simple touch to more intimate instruction. Talia is energetic and forward, actively pursuing Itsuki as her feelings grow. Rose is quiet and gentle, gradually opening up through their interactions. Lupina appears reserved at first but shows a bold side, eventually visiting Itsuki's room to beg him to help her become a goddess.

The early story focuses on Itsuki guiding each sister through a series of increasingly intimate lessons, each prompted by the Stamen Scripture. As the narrative progresses, a rival faction called the Moonlight Faith emerges, seeking to exploit the power of the holy maidens for their own ends. The sisters' growing affection for Itsuki becomes entangled with their duty to protect the world, and deeper secrets about the true purpose of his summoning begin to surface. The series blends explicit sexual content with fantasy world-building, comedy arising from the sisters' innocent reactions, and a harem romantic comedy dynamic that explores the emotional bonds forming between Itsuki and each candidate.