The mood in The Custom of the Country begins lightheartedly and cheerfully, as the early chapters of the book are romantic and comedic. Undine, a beautiful and stylish young woman, is a charming character, and her extreme desire for men and material goods seems almost like an endearing affectation. Early on, it seems like the story might be a simple romance, in which a strong-minded woman finds her way to a man.
Once Undine and Ralph marry, the mood becomes more tragic and emotional. Ralph is very sensitive and thoughtful, loving to read but hating to write. He is always tied up in his own emotions, and dotes on his wife, but even still he is so imperceptive that he never really understands Undine. All this makes Ralph a pitiful figure. Ralph and Undine's marriage, and its ensuing collapse, turns the mood of the book tragic. By this point, Undine's ambition is no longer charming but vapid and unpleasant.
The mood of pitiful tragedy deepens in Undine's flirtation with Peter and her marriage to Raymond. Undine's ambition further sours in the eyes of the reader, from annoying and grating to properly indefensible, when she exploits her son for money from Ralph. As the novel progresses, the mood becomes more dour and concerning.
At the end of the novel, Undine's ambition becomes less of an object of disgust, the mood after she extorts Ralph over Paul, and becomes more of an object of pity again. Even after her marriage to Elmer, when she truly has everything she wants, Undine still longs to be an ambassador's wife, a role that she will never be able to have because of her divorces. At that point, it is clear that Undine can't help it—her rapidly changing desires and ambitions are an avoidable fact of her life. This changes the mood: the reader feels disdain and resignation as it becomes clear that Undine's character will never change, no matter how much her class does.