Shinyû no haji haha: Shiroi shitagi no shimi

親友の恥母 白い下着の染み

Also known as パンティの染み 後妻は淫女, Panty no shimi: Gosai wa injo

Yuya, who harbors a secret longing for Kyoko—his friend's younger sister Yumi's tutor and their stepmother—steals a pair of Kyoko's panties under the pretext of using the restroom. However, Yumi witnesses the act, and he finds himself being blackmailed.