El tren de cuerpo 1

One day the somewhat slender student Akiyoshi takes all his courage and finally reveals his affection to his girlfriend. However due to his slender physique he is unable to end up with her. As a result he decides to properly train his body so that something like that will not happen to him again. When he finds a note saying that construction workers are wanted, he decides to take the job. However he has to take a "practical test" first to see how much "stamina" he actually has. Bit by bit he realizes that this is no normal construction site he was hired to work for but that everything here is about sex...