Ryu is sleeping with his college professor, but doesn't feel loved, so he decides to kidnap his professor's son, Hayato, out of revenge. As it turns out, Hayato's relationship with his father is not very good, so he winds up living with Ryu.
The doujinshi called "Nezi no Kaiten - EX", (done by the mangaka) begins when they start living together. It was released inside the book -6mm no Taboo.