Meijou

One day, a beautiful girl at school, Otsuki Roonue (17), was attacked by two thugs while on a date with her boyfriend, Hoshikai Singhai (17). Her resistance is in vain and Hoshikai, who tried to protect Otsuki, is stabbed and falls into the sea of blood. When the remaining Otsuki is about to be raped, the school principal’s son, Oborozuki (16), appears.
With his help, Otsuki’s chastity is protected, and Hoshikai’s life is saved. However, Oborozuki did not intend to help them for free. Instead, he threatened Otsuki on the condition of “being his woman for a week”, and Otsuki had no choice but to agree to him.

After school, with Hoshikai in the hospital, Otsuki is called to meet with Oborozuki. She begs to release her from the agreement, but discovers how eager he is to take over her body.

(Source: Official site, translated & edited)