One day, Reiko, who lives with her daughter and son-in-law, catches Hikaru, her son-in-law, masturbating due to sexual frustration. Unable to take her eyes off his enormously thick cock, Reiko becomes obsessed. Learning that the cause is her daughter's sexless marriage, she offers to satisfy him instead. Unable to resist his mother-in-law's advances, Hikaru succumbs easily. Since then, the two have secretly indulged in their pleasures behind the family's back.