After leaving Gregoria, they drive through a jungle with "snaky" trees a hundred feet tall. The headlights are broken and they have to proceed in the dark. Realizing that Stan is still high makes them laugh and they feel better. They are exhausted, so in the tiny town of Limon, they park the car by the side of the road and sleep, Dean and Stan on the road and Sal on top of the car. It is extremely hot and humid, and the air is full of insects, especially mosquitoes, which keep dropping on them. A sheriff on horseback comes by in the middle of the night, but only to check that everything is all right; Sal feels that he is a kind guardian of the town. In the middle of the night, like a dream, a white horse dragging a cart comes galloping down the road directly towards Dean, but Sal is confident that the horse won't trample him--and it doesn't.
In the morning, caked with dead bugs, they drive on. The country changes. The indigenous people here are shorter, darker, and stranger to them. They stop on a Sierra Madre mountain peak and look at a three-year-old Indian girl. Dean speculates that all her life, she will know no place but that particular ledge of that particular mountain. They feel their differences strongly. Continuing on, a group of Indian girls selling rock crystals surround the car. They are very young but look old. Dean, being especially kind to one of them, gives her a wristwatch for a small crystal. The girls follow the car as far as they can. They pass through more mountains, a plateau, and then, from the next mountain pass, see the huge sprawl of Mexico City.
Vast, wild, packed with people and ceaseless movement, Mexico City feels to them like the true end of their travels. They walk through the streets "in a frenzy and a dream," Sal describing everything he sees. Then Sal gets dysentery and a fever so bad that he becomes delirious and unconscious. A few days later, he wakes to Dean telling him that he has gotten his divorce and that he is driving back to New York immediately, Stan will take care of Sal. When he gets better Sal realizes what a jerk Dean is, but he understands too that it's just the way he is.
In the concluding section, Sal relates that Dean made it to Louisiana before the car fell apart, and got money from Inez to fly the rest of the way. Once in New York, he reassured Inez about the divorce, then got on a bus to San Francisco and went back to Camille. Sal, back in New York now, has met Laura, the kind of woman he was looking for this whole time. When Dean hears that they are planning to move to San Francisco, he says he will come get them himself, except he comes five weeks early. Dean is barely coherent now, not speaking very much. The visit is rushed, and when they part, Sal has to leave Dean standing on a street corner alone (Sal and Laura are going out with Remi Boncoeur, who, due to past experience, has no patience for Sal's friends). Dean takes the train back to San Francisco; Sal and Laura can't go yet. Sal doesn't know what Dean came for this time, except to see him.
When the sun goes down, and Sal looks over the river to New Jersey, and imagines the vast stretches of land beyond, he thinks of Dean continually.
Mexico City is the end of Sal and Dean's restless wanderings together. Past Gregoria, they have gotten out of their element. They no longer feel an all- embracing humanity with the people they see, but discomfort and difference. The alien-ness of the places forces Sal to observe clearly, objectively, and the descriptions in this section, free from his usual biases, are quite vivid. The air in the Limon jungle has an "unimaginable" softness, and life is "dense, dark and ancient", observations with more depth than his earlier descriptions (of life "pure," "primitive"). Dean commented earlier that he did not drive the road in Mexico, the road drove him, and indeed they are at the mercy of the landscape. The headlights don't work, and then, lying in the humidity and heat and bugs, Sal has a sensation of becoming part of the atmosphere.
The dream and romance of the road has gone and been replaced with something deeper, darker and more complex. Dean doesn't stick around for that, and leaves Sal sick in Mexico City, making Sal realize once and for all the limits of their friendship. In the end, their positions have changed. Sal is strong, confident, in love, and Dean is incoherent and lost. Still, he loves Dean. Dean is the falling star Sal was chasing; Sal will always remember and value the star at its apex, when it presided over a whole world.