Tachibana Yuuki is a high school math teacher who is, to put it nicely, a loser at life. He's not suited to his profession, painfully introverted, unable to establish rapport or gain any respect from his students, unable to socialize and interact without giving a bad impression with his colleagues and superiors. Things only get worse for him when he's ordered to visit the house of Jinguuji Takao, the most promising student the school's ever had. He's incredibly smart and very athletically gifted, good-looking and charismatic, but totally uninterested in school and thus a bit of a problem. Also, his father is a yazuka boss. During the visit, Yuuki is (surprise surprise) raped by Takao. He's forced into continuing a physical relationship, his ties to Takao only tightened when he finds out Takao's very big secret...
I have to say, the secret kind of came out of nowhere (spoilers in the illustrations, but I hadn't bothered to look at the illustrations before reading the book). But the author manages to use it fairly effectively to put to rest most of the irritating problems this kind of story usually has (besides the ones that come from the story being really cliched).
( the pretty pretty pics with just enough story for context )