Part I of the novel opens with Delaney Mossbacher, a middle-class, white American and nature enthusiast, hitting thirty-three year old Cándido Rincón, an illegal Mexican immigrant, with his car as the latter was crossing the road in Topanga Canyon, near Los Angelels, California. Though the Mexican is badly injured, he simply accepts twenty dollars from Delaney and the two part ways. Delaney, extremely shaken up by the incident but confident that the whole affair is over, gets his car fixed and goes back to his highly scheduled, routine life in his neighborhood of Arroyo Blanco. Every morning he makes breakfast for his wife, Kyra, and his stepson, Jordan, Kyra's son from her first marriage, and after sending them safely on their ways, he sits down to write his column, "Pilgrim at Topanga Creek," for the nature magazine Wide Open Spaces. However, the routine is interrupted one morning by a coyote, who jumps the fence around their backyard and steals and kills one of their pet dogs. Shocked and outraged, Delaney believes that this is a result of Arroyo Blanco citizens feeding the coyotes and decides to make a statement at the next town hall meeting. However, the meeting is only supposed to be about discussing the building of a gate at the entrance of the community (an idea which greatly offends Delaney's strong Democratic values), and as a result, nobody pays attention to him.
Kyra Mossbacher, meanwhile, is as work-obsessed as always. She is a realtor with Mike Bender Realty, Inc., often spending too much time overseeing her properties and not enough time with her family. She is particularly fond of the Da Ros place, a mansion of a home with a gorgeous location overlooking the valley. Yet even that property is not enough to lift her dampened spirits, what with the death of her beloved family pet. Meanwhile, Delaney continues his normal routine, writing his column and hiking in the canyon. He also has several conversations with Jack Jardine, a family friend and the President of the Arroyo Blanco Property Owners Association, and other Arroyo Blanco residents such as Jack Cherrystone, who lends his voice to Hollywood trailers, and Jim Shirley, an obese man who is known for spreading grisly tales of crime. Jardine and the others are constantly trying to convince him of he dangers that illegal immigrants pose. On one of Delaney's hikes he encounters the man with the backwards cap and his friend, and he assumes that they are responsible for the litter and mess that he finds on the trail. Angry and frustrated, he hikes back up to where he left his car, only to find that it has been stolen.
Cándido walks away from the accident badly injured, and his seventeen year old wife América finds her collapsed on the side of the road, delusional from his injuries. While he is recovering, she goes day after day to the labor exchange despite his protests, trying to find work to earn money. Both América and Cándido are from the small, poor town of Tepoztlán, Mexico, and they had eloped together after Cándido caught his first wife, who happened to be América's older sister, cheating on him with another man. After some unsuccessful days at the exchange during which she meets Mary, a large white lady with alcoholic tendencies who is also looking for work, and the man with the backwards cap, a man with pale skin and dark brown eyes that reflect his damaged, evil nature. Luckily, she manages to find work cleaning Buddha statues for Jim Shirley. She is overjoyed at having earned money for the first time in her life and at being able to buy food. The couple hits another speed bump when their campsite is attacked by two teenage boys, one of whom is Jack Jardine, Jr., the son of Jack Jardine. Despite their property being ruined, Cándido manages to build them a new campsite further upstream, and soon he is well enough to join América at the labor exchange. With the two of them working, they should be able to realize their dreams of success much faster. However, their lives take a turn for the worse when América is attacked and raped by the man in the backwards cap and his poncho-wearing friend.
In part II of the novel, Delaney's mood begins to shift more and more towards anger and discontentment. The stealing of his car has greatly affected him, and not even hiking can completely take his mind off of it. After picking up his new car, he meets Kyra for an eventful lunch date, during which Kyra confronts four full-grown men after seeing that they locked their dog in their car despite the extreme heat. After driving away from the restaurant, she sees a throng of Mexicans on the corner of one the streets where she works, a sight that bothers her, since she knows that will bring down the property values. She shows the throng to her boss, who uses his connections to get the area cleared. When she is telling this to Delaney, he recalls a previous night, when he met Dominick Flood, a friend of Jack Jardine's and a wealthy businessman who, after getting involved in some shady business deals, is now serving three years of house arrest. During the get together, Jardine, Flood, Cherrystone, and Shirley all expressed the danger that they felt was presented by the immigrants and began hinting that they wanted a wall around the whole Arroyo Blanco community. He is remembering this night while preparing dinner when, all of a sudden, the coyote returns and steal their second family dog.
Devastated, Kyra listlessly goes about her business. One day, as she is closing up the Da Ros place, she runs into the man in the backwards cap and his friend, and it is only her quick thinking that prevents what could have been a dangerous situation. She goes back the next day to find the hateful Spanish words "Pinche Puta" spray painted onto the side of the house. This, combined with her two lost dogs, is enough to convince her to join the campaign to build a wall around Arroyo Blanco. The fact that she has agreed to this angers Delaney greatly - she knows how much he is offended by the idea. He believes that, after specifically picking Arroyo Blanco for its proximity to nature, building a wall would completely negate their reasons for moving there, not to mention the exclusive, racist nature of such a construct. He overhears Jack Jr. saying horrifying things about Mexican woman and feels despair for his community. However, when Todd Sweet, another Arroyo Blanco resident who adamantly opposed the gate and opposes the wall as well, gives him a chance to fight back, Delaney refuses to help, scared of causing more trouble at home. He even goes as far as confronting the man in the backwards cap when he sees him walking across the Cherrystones yard, assuming that he is stealing when he is in fact delivering fliers. Delaney is ashamed of his actions, yet he continues to remain inactive about the wall, which ends up being approved. He remains angry with Kyra for a few more days but, while shopping for their Thanksgiving dinner, he decides to let it go.
Cándido, meanwhile, has forbidden América from going to the labor exchange again, even though she lied to him and told him that the men only robbed her and didn't rape her. She grows increasingly bored in the campsite, even though Cándido, who is managing to find fairly steady work, is constantly bringing her food and presents. Cándido is managing to add a good deal to their savings until, one day, he finds the labor exchange gone. It turns out that Dominick Flood used his connections to shut it down because people were uncomfortable with the throngs of Mexicans who would congregate there. This actually excites América, who believes that this means that they will have to move out of the canyon. However, when the pair go to Canoga Park, an area with a highly concentrated Mexican population that makes América feel much more comfortable, Cándido is robbed and all of their savings are stolen. This, combined with the shame of dumpster diving, sends América into a deep depression. She is angry with Cándido for not delivering on his promises for a better life and she misses her family greatly. Cándido manages to find some work with a man named Señor Wilis despite the closing of the labor exchange, and, in a stroke of good luck, two white men give him a turkey. The site of the large bird is enough to make América smile once again and to convince her to forgive her husband. Their joy, however, is short-lived. The wind picks up a coal from their fire and sets the dry trees and foliage of the canyon ablaze.
Part III of the novel finds the Mossbachers getting ready to go to a Thanksgiving soiree at Dominick Flood's house on Thanksgiving afternoon. While Delaney cannot really enjoy himself, worrying about the Thanksgiving dinner that they left cooking at home under the supervision of their Mexican maid, Kyra has a great time, as does her mother Kit, who is visiting for the holiday. Kit immediately senses Dominick's bachelorhood and clings to him for the entire party. Delaney and Kyra are talking with the Jardines about the wall when suddenly someone shouts that there is a fire in the canyon. All of Arroyo Blanco ends up being evacuated, and while standing on top of the canyon away from the flames, the man in the backwards cap and his friend come trudging up towards them. Delaney, who has been drinking and is worried and angry, shouts for the police to arrest the two men, as he is convinced that they are responsible for the fires. He soon incites a riot, with the whole community shouting racial slurs at the two Mexicans. While they are being arrested, the man in the backwards cap spits in Delaney's face, and Delaney attacks him. The next morning, when they are moving back into their homes, Delaney feels extreme shame and regrets his action. However, before he can dwell on it for too long, the family discovers that not only is their cat missing but also that Dominick Flood had taken advantage of Kit, cutting off his house arrest ankle bracelet and putting it in her purse, giving him time to flee the country. To top everything off, the fire had destroyed the Da Ros property, devastating Kyra. While she and Delaney are out on a walk in an attempt to shake off everything that happened, they run into Jack Jardine, who shows them that the wall by the front gate of Arroyo Blanco has been vandalized. Delaney decides to set up some trip-wire cameras and to stake out the wall in an attempt to find the guilty person. He spends weeks staking out the area, completely disrupting his normal routine in an attempt to catch the culprit. On the one night that he takes off, one of the cameras is tripped, and, to his surprise, the face of Cándido, the man he hit with his care, is what is captured.
Cándido and América, meanwhile, had survived the frantic climb out of the canyon to escape the fire. Once they are out on the road, América, back in her despair, goes into labor. In his haste to find a safe place for her to give birth, Cándido finds the newly built Arroyo Blanco wall, specifically the section right behind the Mossbachers' house. He cannot get over the wall, but he finds an aluminum shed where América can stay. Cándido is beside himself with worry, knowing nothing about delivering a baby, but América manages to get through it with the help of none other than Dame Edith, the Mossbachers' pet cat. Finally, the Rincón's beautiful baby girl, whom America names Socorro, is born, and the danger posed by the fire is averted. While his wife and child sleep, he is forced to subvert his values. He scales the Arroyo Blanco wall and steals little things, like some tools and food, from the back yards behind the wall. Using these materials, he builds a new hideout for his family on top of a ledge overlooking the canyon and transports his wife and child there. América is now constantly telling him that she wants to go home so that she can raise their child in a civilized environment. She is also telling Cándido that their child needs to see a doctor, for she thinks that Socorro might be blind. Cándido is now constantly wracked with guilt over the fact that he cannot provide for his wife and child, and his attempts to get work become more and more desperate. It gets to the point that he is standing on the side of the road trying to find a job, not caring if he attracts the attention of immigration officers. It is there that he is spotted by Delaney.
Upon seeing Cándido on the side of the road, Delaney immediately pulls over and yells at him to stay where he is, calling 911 and intending to turn the Mexican in to the police. Cándido cannot allow this to happen and runs into traffic to get away, causing a truck to swerve and to hit Delaney's improperly parked car. Delaney could care less, only waiting long enough for the tow truck to pick up his car before he takes off after the Mexican. By now, he has convinced himself that Cándido is responsible for all of the vandalism, stealing, and even the fire, and, in a haze of anger, he tracks the Mexican's footprints. The trail goes by the Arroyo Blanco gate, and Delaney sees that the wall has been vandalized once again and that both of the trip-wire cameras have been triggered. He quickly goes home to shower, eat, and grab his gun, and then develops the film. To his surprise, it is Jack Jr.'s face that is caught on the film, not Cándido's, but this does not stop him from going after the Mexican. Delaney follows the footprints to Cándido's makeshift hut, where América has just told him that Socorro is blind and the truth about the rape in the canyon. As Delaney bursts in on them, carrying a gun, all of them are suddenly swept away by a raging river. In the deluge, América and Cándido lose Socorro, and yet, despite his despair, when Cándido sees Delaney struggling in the water, he reaches his hand out to save him.