Shy boy Shintaro discovers a mysterious application on his phone with the power to transform real women into monster girl fantasy creatures. His childhood friend Kanako volunteers to help him test the app, and through their experiments they grow closer, eventually starting the romantic relationship she has always wanted. The main storyline follows Shintaro and Kanako as he uses the app to turn her into a different monster girl each chapter: a soft-winged harpy, a mermaid in a school swimsuit, a tentacled scylla, a lamia, an arachnid girl, and others. These transformations lead to creative sexual encounters that take advantage of each monster form’s unique anatomy, such as harpy egg laying, scylla tentacle play, and spinneret-based intimacy. The main couple’s dynamic remains largely static after the first chapter, and the story gestures toward a larger mystery behind the app that is never resolved. Interspersed with the main narrative are standalone chapters featuring different characters in a world where monster girls simply exist: a descendant of Kiyohime who is self-conscious about her serpent lower half, a slime girl who becomes a sexy teacher, an oni girl in black tights, a young man having sex with the anthropomorphized form of his pet cat, and others. These one-off stories vary in inventiveness, with some leaning into familiar cat-girl tropes while others explore more unusual pairings. The setting is a contemporary everyday environment where the supernatural app or the natural presence of monster girls creates opportunities for erotic encounters. There is no central conflict beyond the protagonist repeatedly transforming his girlfriend, and the narrative focuses thoroughly on exploring the variety of kinks and physical possibilities each monster type affords, without introducing significant plot progression or character development for the leads.