On the afternoon of March 7, 1965, Alabama state troopers and members of a Dallas County posse, armed with clubs, cattle prods and tear gas, attacked civil rights demonstrators on the Edmund Pettus Bridge in Selma, Ala. The marchers had planned to walk the 50 miles to Montgomery, the state capital, as part of a long-building protest against the denial of basic voting rights to Southern blacks. The procession would have crossed Lowndes County, where not a single African-American voter had been registered in more than 60 years. Efforts to change this had been met with bureaucratic obstruction, intimidation and lethal brutality, including the killing, a week earlier, of Jimmie Lee Jackson, a 26-year-old laborer and protester, by a state trooper.

A few days later, a second march, led by the Rev. Martin Luther King Jr., turned back rather than risk further violence. By the time the third, ultimately successful effort left Selma on March 21, President Lyndon B. Johnson, pushed by King and televised images of official brutality as well as by his own political and moral instincts, had introduced the Voting Rights Act in a nationally televised address to Congress.

The above can serve as a partial plot summary of "Selma," Ava DuVernay’s bold and bracingly self-assured new movie about the march and the events, in Selma and elsewhere, leading up to it. History does not come with spoilers — or spoiler alerts — but it does have a habit of setting traps for ambitious filmmakers. The most basic is the lack of clear beginnings and ends. To tell the story of the American civil rights movement properly, a conscientious director would have to plot a course from at least 1619, when the first African slaves arrived, to this week.

Narrowing the scope — "Selma" begins with King’s acceptance of the Nobel Peace Prize in Oslo in December 1964 and ends in Montgomery a little more than three months later — poses its own challenges. How do you capture the chaos, uncertainty and sheer crowdedness of events without sacrificing coherence? How do you endow a relatively well-known episode from the recent past with the urgency of the present tense?

The answers are all up there on the screen. DuVernay, in her third feature (after "I Will Follow" and "Middle of Nowhere"), writes history with passionate clarity and blazing conviction. Even if you think you know what’s coming, "Selma" hums with suspense and surprise. Packed with incident and overflowing with fascinating characters, it is a triumph of efficient, emphatic cinematic storytelling. And much more than that, of course: It would be hard to imagine a timelier, more necessary popular entertainment in the year of Ferguson, Mo., a reminder of progress made and promises unkept. But such relevance is hardly automatic. A timid, pious or dishonest movie about the time-burnished glories of the civil rights era — the kind of soothing fable of awakened white conscience that Hollywood has too often favored — would not do anyone any good.

Instead, DuVernay, working from a screenplay credited to Paul Webb, has stripped away layers of fond memory and retroactively imposed harmony to touch the raw, volatile political reality of the mid-1960s: the courage and the cravenness, the idealism and the calculation, the visible and invisible divisions and rivalries. King, played by David Oyelowo with the requisite grace and dignity and also with streaks of humor, weariness and doubt, occupies a central place in "Selma," but the film is less interested in affirming his greatness than in understanding its sources and limitations, and in restoring his human dimensions.

King’s heirs did not grant permission for his speeches to be quoted in "Selma," and while this may be a blow to the film’s authenticity, DuVernay turns it into an advantage, a chance to see and hear him afresh. Oyelowo, a British actor of Nigerian background, has mastered the Southern inflections and preacherly cadences that have become part of the permanent soundtrack of our education system, and the script offers credible paraphrases of his character’s eloquence. But this version of King is also, in his mid-30s, a man trying to navigate his public role, his private life and the expectations of his allies and friends.

DuVernay’s portrait is astonishingly rich and nuanced, and it is very much a group portrait. We see King at home in Atlanta with his wife, Coretta Scott King (Carmen Ejogo), confronting her worries about his safety and her suspicions of his infidelity. Those suspicions, handled with delicate candor, are fueled by the determination of FBI chief J. Edgar Hoover (Dylan Baker), to destroy a man he calls a "political and moral degenerate."

Under constant surveillance — FBI field reports typed onto the screen fill the audience in on his whereabouts — King travels a hectic circuit from pulpit to jail cell to the White House and back. There are many piquant moments of behind-the-scenes political theater in Washington and Montgomery, but while the film acknowledges the importance of men in power, its heart is with the activists on the ground in Selma.

To say that extraordinary actors support Oyelowo’s per­for­mance — Oprah Winfrey, Wendell Pierce, Tessa Thompson, Henry G. Sanders and many more — is to state the obvious and also to get it backward. King worked in the service of the movement, not vice versa, and Oyelowo’s quiet, attentive presence upholds this democratic principle by illuminating those around him.

"Selma" seems to contain the seeds of at least a dozen other movies — a reminder of how fertile the civil rights era is and how poorly it has been explored by popular culture. How long will we have to wait for a biopic devoted to Bayard Rustin (Ruben Santiago-Hudson), Amelia Boynton (Lorraine Toussaint) or Andrew Young (Andre Holland)? Or for a daring director to tackle the story of the Student Nonviolent Coordinating Committee? Where is the pay-cable miniseries about Diane Nash (Thompson) and James Bevel (Common)? The Martin Luther King-Ralph Abernathy (Colman Domingo) buddy movie? The "Law & Order" spinoff about the career of Justice Department lawyer John Doar (Alessandro Nivola)? They are all here, in miniature, but "Selma" makes one impatient for more.

Impatience was one of King’s virtues: His "Letter From Birmingham Jail" was published as a pamphlet titled "Why We Can’t Wait." "Selma" takes up his history with its eyes on the future, reminding us that the voting-rights victory nearly 50 years ago was not inevitable and is not yet complete. The nonviolent fight against white supremacy required not only righteous vision but also strategic insight and tactical discipline. The ideology that would sanction the beating and killing of black Americans who dared to assert their citizenship has not vanished, though its methods, language and partisan affiliations have changed.

"Selma" is not a manifesto, a battle cry or a history lesson. It’s a movie: warm, smart, generous and moving in two senses of the word. It will call forth tears of grief, anger, gratitude and hope. And like those pilgrims on the road to Montgomery, it does not rest.

© 2015 The New York Times Company