Yuzuki Yukari is a Voiceroid, a piece of software you can use for text-to-speech. In this doujin, she also has a physical body.
After the main character uses her for a whole month to voice porn to him, that physical body gets a bit hot and bothered. And when an update adds sex functionality (why did it didn’t come out of the box is anybody’s guess), things are primed to escalate quickly.
For a day, Yukari is not a Voiceroid serving her master, but rather an employee at a certain establishment, servicing her customer. Of course, this is all pretend play, but variety is the spice of live and time spent in the bathroom with a horny Yukari is very spicy indeed.