Night Shift Nurses (夜勤病棟 Yakin Byoutou) is a Japanese erotic visual novel by Mink that has spawned a hentai anime series in 2000. The anime was released as ten censored OVAs (Original Video Animation), with two recap episodes (Episode 5.5 and Episode 10.5). The first five were released uncensored in North America in 2002 along with recap Episode 5.5. It is somewhat infamous for its very extreme depictions of sexual fetishes including scat, bondage and torture. Several sequels of this series were made: two official ones, three OVAs and a live-action movie. However, only the first ten episodes can be considered canon.
Plot: Night Shift Nurses opens with Dr. Ryuji Hirasaka, a gynecologist, looking back on an event feature everyday objects and human wastes. In most cases, it is not until the second or third sexual encounter that the nurses are 'transformed' and succumb to whatever Hirasaka subjects them to. It is worth noting that, when compared with other hentai productions, Night Shift Nurses is somewhat different - that is, it is also a psychological drama. While Ryuji is indeed despicable, his 'victims' are unable to resist him and, later on, begin to visibly crave for his so-called experiments (aka. his infamous sexual plays).
(source: Wikipedia)