Hikari, a busty sister-in-law, comes to stay at her brother-in-law and his wife's house to sightsee. While living together, her brother-in-law can't help but be curious about Hikari's breasts. He resists the temptation to push her down, but one day he can't hold back any longer and, sneaking away from his wife's eyes, locks Hikari in a secret room and forces her into a relationship. Hikari feels bad for her, but she can't help but feel pleasure.