Katie Grant, is having an affair because she feels her husband, David, doesn't love and appreciate her enough. She wants a divorce and tells David that she wants one, but David says no. Even when his wife tells him she's slept with another man, David refuses to acknowledge the fact and simply continues living their life with their two children and their jobs and the everyday things that they do. This is how "How To Be Good" begins. There is no specific point in the story when things start to change, but they do, and in exceptionally curious and wondrous ways, and Hornby receives them all with a grain of salt, a lime, and the ability to throw them back one after another.
Katie believes she is a good person for the most part; a compassionate doctor, a loving mother, just perhaps not the most reliable wife, but not for inexcusable reasons. David is, by profession, an angry writer, and therefore finds the frustrating and annoying points in all situations. Katie is fed up with arguing and the fights and the lack of understanding, but like David, she doesn't really want things to change profoundly, just enough to be happy.
But things do change. Stephen, the man with whom Katie chose to break her wedding vows, continues to pursue her despite Katie's rebuttals, and even has a serious 'heart-to-heart' with David before finally getting the hint he might be a fling and not 'the one' after all. Meanwhile, David begins collaborating with a healer named GoodNews, who can not only cure bad backs and eczema, but eventually begins to affect David's whole perspective on the world. David quits his job, GoodNews moves in with family, and life is different--not necessarily better or worse, but different. Through neighborhood campaigns for adopting the homeless, elderly patients wanting to live with the family, Katie moving out temporarily, and finding out two people can love each other again (at least enough to stay together), the reader learns it is not so bad to be good.