Darius is a second son of an earl. He's trying to take care of his family the best he can, since his father's worthless. His choice of occupation isn't exactly honorable. When he's hired to help an old man and his young wife conceive, he falls for the wife, which was the old man's idea in the first place. 
 The old man was still in love with his first wife and only married Vivian to keep her safe from her scheming stepfather. Knowing he was dying, he plotted to get Darius and Vivian together and protect his family's inheritance. 
 Of course, there's stuff that has to be overcome. Darius has to cut ties with his previous occupation; the women aren't willing to lose their plaything. He also has to get Vivian's stepfather out of the way. His answer to both these problems isn't exactly gentlemanly, but satisfyingly effective. 
 The first in a series, but it reads strangely, like there should have been parts that we already knew. Things are mentioned in passing. It's a bit confusing when people you haven't really been introduced to conveniently die and loose strings are tied up too easily. All I can assume is that some of those details are filled in with other installments. Also, you might need a dictionary for some of the terminology that's not much used these days.