He gets excited by his girlfriend's defenseless sister and plays a naughty prank on her! While she's not looking, she plays with her clit through her panties! He strokes her, pinches her, kneads her... the stain spreads! She doesn't even realize that what she's being done to her is "naughty" and she cums unconsciously and with embarrassment! He persuades his little sister, who has little sexual knowledge, to reveal her forbidden sexual desires!