Maruhi sennyû: Gyaku soap tengoku

(秘)潜入 逆ソープ天国

Also known as 藤小雪の逆泡踊り天国, Koyuki Fuji no gyaku awa-odori tengoku, Secret Infiltration Reverse Soap Heaven, Fuji Koyuki's Reverse Bubble Dance Heaven

A university assistant professor spends his days researching. A hidden camera is installed at a reverse soap shop run by a friend of his, and he studies techniques by observing the reactions of female customers...