Seiko is a "net seiyuu", a girl who does voice acting work from home for various doujin eroge producers. After voicing a pure love sex scene she accidentaly recognizes Yusuke, one of her classmates, as being another famous "net seiyuu", and her partner in that scene.
From there, each new job she takes on deals with more and more extreme situation, which make her feel more self-conscious about her relationship with Yusuke.