In the words of this doujinshi’s author:

“She loved her boyfriend, but after a night of drinking, her senpai invited her to his apartment. Already pretty tipsy, it wasn't hard for him to turn the night into a night of lustful steamy sex! By the time the sun came up, she has turned into a personal sex-slave behind her boyfriend's back!”

Sounds about right.
*All characters depicted in this work of fiction are adults, regardless of their visual representation or textual description, and are not based on any real person*