It's the narrator's birthday and he's waiting for the mail to come in the hopes that his family has sent him money. Holly happens to walk by and she invites him to go horseback riding in the park. Although she's pregnant and isn't "out to lose the heir" (13.2), she wants to say good-bye to a horse named Mabel.
This is the first the narrator has heard of Holly leaving, and it turns out José has arranged for them to head to Brazil the following week. This news hits the narrator hard, and as they're on their way to the park, he feels like he's "flying, desolately floating over snow-peaked and perilous territory" (13.5). He tells Holly that she "can't really run off and leave everybody" (13.6). Holly tells him that no one will miss her, but he tells her that he'll miss her, and that Joe Bell and Sally Tomato will miss her too.
It seems that Sally takes the news a little better than the narrator does, and he even tells Holly that she should go "because sooner or later there might be trouble" (13.9) (remember the "weather reports" she delivered to Sally? We're guessing it has something to do with this). He even gives her five hundred bucks as a "wedding present" (13.9) (does this sound like a payoff to anyone else but us?).
The narrator, clearly upset at the thought of Holly leaving, "want[s] to be unkind" (13.10), so he questions whether or not the wedding will actually happen and then brings up Holly's first husband, Doc. She warns him about telling anyone about Doc as she "rub[s] her nose" (13.15) (remember that she does this when she's getting upset or uncomfortable with a too-personal conversation).
They finally get to the stables and mount their horses. Holly chooses one for the narrator that is "safer than a cradle" (13.16), and as they ride through Central Park the narrator starts to feel exhilarated and happy: "Suddenly, watching the tangled colors of Holly's hair flash in the red-yellow light, I loved her enough to forget myself, my self-pitying despairs, and be content that something she thought happy was going to happen" (13.18).
As he's basking in these warm feelings and enjoying the horseback ride, a group of boys jump out of the bushes and scares his horse. The narrator reveals himself to be a bit of a racist (or at least to subscribe to some pretty racist stereotypes) when he describes the boys as a "band of Negro boys" (13.19) who act "like savage members of a jungle ambush" (13.19).
The narrator's horse freaks out, stands on her hind legs (with him still on her), and then takes off running so fast that "her hooves made the gravel stones spit sparks" (13.20). Holly races after the narrator as his horse carries him to Fifth Avenue, "stampeding against the noon-day traffic, taxis, buses that screechingly swerved" (13.20). Holly keeps after them, and a policeman joins the chase as they maneuver their own horses on either side of the narrator's. This stops the horse suddenly and the narrator falls off, not really clear where he's ended up.
After the policeman offers to take the horses back to the stables, Holly and the narrator get in a taxi and she is really, truly worried about him. "You might have been killed" (13.27), she tells him, her "face […] white with concern" (13.26). The narrator's a little embarrassed by the whole situation, but more than anything he's thankful that she stopped his horse: "But I wasn't. And thank you. For saving my life. You're wonderful. Unique. I love you" (13.28). Holly kisses the narrator's cheek when he says this, and then he promptly passes out.