Nana to Kaoru: Koukousei no SM Gokko

Kaoru is in love with his childhood friend turned idol, Nana. Over the years, their relationship has become strained with the differences in their lifestyles, but Kaoru still fantasizes about Nana helping him realize his ultimate fantasy... a BDSM relationship with her! One day, Kaoru's mother finds his toys and gives them to the ever responsible Nana to rid her son of his fetish, but Nana finds a leather piece and accidentally locks herself inside! This moment awakens something inside of Nana, whose stressful life could use some fun and could be the start of Kaoru's ultimate fantasy... 

(Source: Denpa)