After a marital fight causes his wife to return to her parents' home, Koji is left feeling depressed. His sister-in-law, Mako Oda, comes to check on him out of concern. Opening up to her gentle and attentive listening, Koji gradually lets his guard down. However, he soon realizes Mako's true intentions lie in seducing him. That night, in the bath, Mako touches his back, and as Koji feels the softness of her big tits against him, he becomes flustered—yet drawn to her as someone who fills the emptiness in his heart.