Nana to Kaoru: Black Label

Nana to Kaoru: Black Label series begins 8 months in the future of the parent version, with Nana and Kaoru in the summer of their final year of high school.

Nana is a normal teenage girl in her last year of high school getting ready for college admissions tests during her summer vacation, with the exception of one thing: She takes "breathers" with her childhood friend, Kaoru. An encounter with Tachibana, a worker she knows at an S&M shop, brings Nana further into the S&M world as she learns what it means to be master and slave.

(Source: MangaHelpers)