David is one of the good guys from next door, a genuinly nice and normal human being. Except that he has bought a house under a false name for him and Annabelle, who in his mind is to become his wife rather sooner than later. Problem: Annabelle is married to another man and she isn´t answering his letters. A minor nuisance which might resolve itself some way or another.
Well, it is a Patricia Highsmith novel, so I except "the one way or another" will turn out to be psychologically interesting. It´s just an inkling, though.