An all-nighter spent skimming Plato’s “Republic” would be a less hurried undertaking than “What Is Democracy?,” a hugely ambitious documentary from the Canadian director Astra Taylor (“Zizek!”). Like democracy itself, the movie assumes such a broad mandate and has such noble intentions that indicating its shortcomings seems almost beside the point.

The overarching concept here is to explore the philosophical underpinnings of democracy by talking with scholars and visiting the present-day sites of Plato’s Academy and the Agora in Athens. Using quotations from “The Republic” as glue, Taylor travels elsewhere and finds imperfections in the democratic process.

In the United States, Cornel West points out instances when majority rule hasn’t always been desirable, noting that the Emancipation Proclamation and Brown v. Board of Education both arrived by fiat. Many of the film’s most stinging moments deal with the degree to which true democracy has rarely if ever existed for African-Americans.