He's someone she admires, someone who knows both the world of music and the world of adults. Mio's heart is easily seduced by his reliable demeanor and sweet words. At first, he's just someone to talk to. But before she knows it, their friendships have turned into secret rendezvous, and Mio begins to dress up in all sorts of cosplay costumes at his request.
In front of the camera, she does as she's told, and spends her days "acting" as the heroine of a doujin adult video. The faces of the friends she once played music with have now completely vanished from her life. This is a record of the twisted sense of security and pleasure that a young girl gained in exchange for her own weakness.