The novel follows two couples living in Los Angeles - the wealthy white American Delaney and Kyra Mossbacher, and the undocumented Mexican immigrants Cándido and América Rincón, who are struggling to survive.
One day Delaney accidentally hits Cándido with his car, leaving Cándido injured. This initial encounter establishes the conflicts between the two couples.
Cándido and América are living in a camp in Topanga Canyon while looking for work. América is raped by another immigrant, leading Cándido to beat the attacker out of anger.
The Mossbachers' community installs a gate and debates building a wall to keep out immigrants like Cándido. This exacerbates tensions between the privileged residents and the immigrant workers. Kyra becomes obsessed with protecting a house she is selling after it is vandalized, likely by immigrants. Delaney grows increasingly paranoid about crime.
In the climax, a mudslide hits both couples, killing Cándido and América's daughter. Despite their conflicts, Cándido tries to save Delaney from the mud.