[Warning: contains spoilers!]

I've watched some trailers almost by mistake, and it appears to be a deep story

A high-school boy confesses his feelings to the quiet, enigmatic Katase Hina on the school rooftop, only to be gently rejected. Unable to forget the strange, pained expression on her face as they parted, he follows her home one night and witnesses a shocking scene: Hina is being sexually abused by her elderly grandfather. Traumatized by earlier incidents, Hina has become physically insensitive (anorgasmic) to sexual stimulation, a condition the grandfather exploits while claiming to "cure" her with persistent abuse, often using large amounts of lubricant to simulate arousal and produce sounds.

Devastated but determined, the boy confronts the situation indirectly. After Hina reveals her tragic backstory—orphaned parents, dependence on her grandfather despite the abuse—he confesses again and begins a secret relationship with her. They plan to endure until graduation, when she can escape. The grandfather grows suspicious and escalates his control, but during one assault, Hina suddenly begins to feel pleasure again—possibly triggered by her new emotional bond or the extreme situation. In a dramatic confrontation, the boy intervenes; the grandfather suffers a cerebral hemorrhage and dies shortly after.

Afterward, the boy convinces his parents to take Hina in until graduation under strict conditions. Once living together, Hina gradually regains full sensitivity, leading to a consensual, loving intimate relationship. The story ends on a hopeful, affectionate note as the couple builds a future together.