Despite her boyfriend being nearby, the girl Haruna has secretly liked for a long time suddenly starts making advances on him under the kotatsu, out of his sight. Caught off guard, I panic—but from her behavior, it seems she doesn't mind at all. Gradually, Haruna becomes more receptive, and the two begin a series of secret, lewd encounters behind her boyfriend's back. This naturally escalating erotic relationship begins to spark unexpected feelings between them.