During her husband Seiichi's business trip, his father, Fumiki, comes to stay in his place. Taking advantage of her husband's absence, Fumiki forces himself on Tsukasa. Initially resisting, she gradually finds herself captivated by her father-in-law's massive cock—something her husband could never compare to. Over the course of the week-long trip, she is repeatedly taken by him every day, slowly transforming into a slave for his huge cock. As her relationship with her father-in-law deepens, Tsukasa experiences pleasure and sensations unlike anything she's ever known.