Sexually frustrated by a sexless marriage, Mariko secretly masturbates through her lonely days. Then she discovers that her live-in son-in-law Shinji is also sneaking off to masturbate for the same reason. Captivated by the freakishly huge cock she glimpses by chance, Mariko tells herself she is only helping him get relief in her daughter's place. Day or night, she secretly tempts him at every opportunity. Unable to resist the mature sensuality pouring from his mother-in-law, Shinji lets her toy with his cock again and again.