Shinji Nagumo, a high school student, faces a dilemma with his standoffish step-sister, Tsumugi. Their distant relationship takes an unexpected turn when Tsumugi introduces her 'net friend'—the popular Yua Takarai, who turns out to be Shinji's classmate. As the trio spends time together, Tsumugi starts to open up, and Yua becomes unusually close to both Shinji at school and in their shared home. Despite Shinji finding Yua challenging, he discovers a softer, lonely side to her that only he can see. This revelation leads to a surprising proposal from Yua about living together as a trio. What begins as a misunderstanding of Yua's intentions as Shinji's lover evolves into a heartwarming and comedic tale of companionship and romance between the two girls—a gal and a step-sister sharing warmth and sweetness in their growing bond.