The narrator meets Holly late one night after seeing her in the halls of the apartment building a few times before. She knocks on his bedroom window and he lets her into his apartment and into his life.
The two start spending a lot of time together and the narrator comes to regard Holly as pretty important to him. When he doesn't get to see her, he misses her presence a great deal.
Things seem to be going along fine until the narrator finds out that one of his stories is going to be published. When he races to her apartment to tell Holly about this exciting news, the two end up having a nasty argument since she totally misunderstands and then criticizes his writing. She throws him out of her apartment and he vows to never speak to her again.
He keeps his promise for a while, but then he meets Doc Golightly, and when he subsequently learns more about Holly's childhood he feels a bit more inclined to forgive her.
The neighbors resume their friendship, but then the narrator learns that Holly is planning to leave New York and move to Brazil with José. This throws him into deep sadness since he realizes that he's never been part of the plans she has for her life.
Before she leaves, Holly takes the narrator horseback riding and the outing's a catastrophe. His horse freaks out after being scared by a group of boys and takes off running out of the park and onto Fifth Avenue. It's only after Holly and a policeman catch up to him that the narrator's horse stops, and in a cab on the way home he passes out from the shock.
After Holly gets arrested and decides to leave the country, the narrator helps her get her stuff together, even though he's still devastated that she's leaving. On the way to the airport she drops her cat off in a strange neighborhood, for which the narrator roundly criticizes her. When she realizes the mistake she has made and unsuccessfully tries to find her cat, he promises to find the cat for her and to take care of it in her absence.
He does eventually find the cat leading a seemingly happy life in a new home. We learn that he sells a few more stories and eventually moves out of the apartment building because it has too many memories of Holly. As the novel reaches its conclusion, the narrator gets the aforementioned postcard from Holly and we're left with his final thought: that he hopes she has found a place where she belongs.