DuVernay contends that slavery in the United States has been perpetuated since the end of the Civil War through criminalizing behavior and enabling police to arrest poor freedmen and force them to work for the state under convict leasing; suppression of African Americans by disenfranchisement, lynchings, and Jim Crow; politicians declaring a war on drugs that weighs more heavily on minority communities; and, by the late 20th century, mass incarceration affecting communities of color, especially American descendants of slavery.

There follow interviews with a number of activists, academics, political figures from both major U.S. political parties, and public figures, including Angela Davis, Bryan Stevenson, Michelle Alexander, Jelani Cobb, Van Jones, Newt Gingrich, Cory Booker, and Henry Louis Gates Jr..[6] The economic history of slavery and the post-Civil War racist legislation and practices that replaced it are explored.

In addition, Jim Crow legislation was passed by Democrats to legalize segregation and suppress minorities, forcing them into second-class status.

Following the passage of civil rights legislation in the 1960s that restored civil rights, the film notes the Republican Party's appeal to southern white conservatives, including the claim to be the party to fight the war on crime and war on drugs, which began to include mandatory, lengthy sentencing.

A new wave of minority suppression began, reaching African Americans and others in the northern, mid-western, and western cities where many had migrated in earlier decades.

After their presidential candidates lost to Republicans, Democratic politicians such as Bill Clinton joined the war on drugs.

DuVernay ends the film with graphic videos of fatal shootings of black people by police, which Manohla Dargis describes as, following the previous discussion, having the effect of "a piercing, keening cry.

[10] On the film review aggregator website Rotten Tomatoes, 97% of 109 critics' reviews of the film are positive, with an average rating of 8.8/10; the site's "critics consensus" reads: "13th strikes at the heart of America's tangled racial history, offering observations as incendiary as they are calmly controlled.

Summarizing the film, Dargis wrote that "The United States did not just criminalize a select group of black people.

On a panel about the future of film published in The New York Times, DuVernay said: I'm told by the system that [a theatrical release] is what matters, but then people aren't seeing your movies.