Nana to Kaoru (2011)

Kaoru is a 17-year-old virgin who has an SM fetish. He secretly dreams about an SM relationship with his childhood friend Nana. One day Kaoru's mom asks Nana to hide all his SM toys so he'll study for a change. However, Nana finds the leather one-piece that Kaoru bought and tries it on, but she accidentally locks it and doesn't have the key! The beginning of super strange relationship.