Kota's father remarries, making Shiori his stepmother. After the birth of his younger brother Daisuke, their relationship begins to change. Feeling unwanted and insecure about not being her biological son, Kota impulsively tells Shiori, "Let me drink your breast milk too," intending only to make her uncomfortable. But when she refuses, Kota realizes something profound—he's no longer seeing Shiori merely as a mother figure, but as a woman he deeply desires. As emotions intensify and boundaries blur, their forbidden bond leads them into a secret world of lactation, passion, and creampie-filled intimacy.