Based on a game by Mink's M no Violet division. 
 
Before Ren Nanase was ever a night shift nurse, she was a sexy coed at a nursing school. Hearing rumors that one of the faculty is a rapist, she does exactly what any model citizen would do and confronts him about it. Unfortunately, things go downhill from there, and she winds up being his sex slave, getting a crash course in depravity. But in the middle of that perversion, could there be a hint of purity? 
Note: There are 2 Nanase Ren OVAs produced totally different studios, but both feature the same character. The other one is the "Kranke Nanase Ren" (夜勤病棟　Kranke　七瀬恋) bit, which is a special for Yakin Byouto

On a forbidden hospital ward, the head doctor performs madly erotic experiments, stripping the nursing staff of their uniforms, inhibitions and self-control... until even he is shocked by what he uncovers!

A side story from the original: Ai wishes to be a real woman, after thinking that she had dreamed her sister doing with the doctor. On the other hand, Ryuji Hirasaka wants to have his pervert experiments trial on Hikaru. Instead, the doctor become a savior to Hikaru &amp; Ai for his new invention having both sisters doing it to one another and Ai thought that it as a dream... The last episode ends as the doctor and his newly wedded bride nurse, Ren, went exile to a village to continue his medical practice... and of course his experiment only on his bride.
 
(Source: Anime News Network)

Before Ren Nanase was ever a night shift nurse, she was a sexy coed at a nursing school. Hearing rumors that one of the faculty is a rapist, she does exactly what any model citizen would do and confronts him about it. Unfortunately, things go downhill from there, and she winds up being his sex slave, getting a crash course in depravity. But in the middle of that perversion, could there be a hint of purity?