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 sets up 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 her 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 an expensive home she's selling after it's 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.

Immigration and the American Dream
The novel contrasts the struggles of illegal Mexican immigrants Candido and America trying to achieve the American Dream with the privileged lives of wealthy Americans Delaney and Kyra. It highlights the harsh realities and exploitation faced by undocumented immigrants in their pursuit of a better life. The "tortilla curtain" itself symbolizes the divide between these two worlds.

Racism and Xenophobia
Boyle examines the racism, prejudices, and fear (xenophobia) that many Americans harbor towards Mexican immigrants, viewing them as a threat to their way of life. The novel exposes the hypocrisy of professing liberal values while still exhibiting racist attitudes.

Social Class and Inequality
The stark divide between the upper-middle class whites and the impoverished Mexican immigrants lays bare the deep economic inequalities in American society. Their conflicting worldviews and access to resources are shaped by their social class.

Human vs Nature Conflict
The novel juxtaposes the pristine gated community of Arroyo Blanco with the harsh natural environment of the canyon where the immigrants live. This explores humanity's complex relationship with nature and the desire to control the wilderness.

Gender Roles
Boyle contrasts the more progressive gender roles of Delaney and Kyra with the traditional machismo attitudes of Candido towards his wife America working.

In essence, The Tortilla Curtain provides a nuanced critique of the American Dream and exposes the nation's struggles with immigration, racism, inequality, and environmental issues.