In a way, the most important review of “By Any Means” is a synopsis, because the film, as a title card declares, is “based on true events,” and the events themselves are unspeakable and need to be widely known. The story is set mainly in Hattiesburg, Mississippi, in 1966, and is centered on the killing, that year, of a Black businessman and civil-rights activist, Vernon Dahmer, Sr. (played here by Giancarlo Esposito), who was targeted by the Ku Klux Klan because of his efforts to register Black voters. More generally, the movie, directed by Elegance Bratton, is about the historical context of this murder, the conscious and organized effort by white Southern citizens and officials to intimidate, persecute, and subjugate Black people. The action is rooted in that racist reign of terror—and the fear that it sowed. Thus, the film, both as a history lesson and as a flash point of current politics, takes on a great responsibility and a grave one.

The local F.B.I. office, in Jackson, Mississippi, begins to investigate the killing, but some locals are too afraid to tell what they know, others too full of hate, so the F.B.I. decides to make use of Gregory Scarpa, Sr. (Mark Wahlberg), a white mobster and hit man from Brooklyn who was serving as a government informant. (Scarpa’s involvement, though widely reported, hasn’t been officially acknowledged by the F.B.I.) In 1964, Gregory had helped the F.B.I. locate the bodies of three slain activists—James Chaney, Andrew Goodman, and Mickey Schwerner—and “By Any Means” opens with a view of the assistance he provided: he ties a white man to a chair and tortures him, beating him with a baseball bat.

Tasked with finding out who killed Vernon, Gregory is accompanied by a Black F.B.I. agent named Wayne Strider (Yahya Abdul-Mateen II), who’s assigned to work as his handler. (This character is a composite of various agents, whereas Scarpa, who died in 1994, is entirely real.) The film is dominated by the two men’s relationship. Wayne, a native of Hattiesburg who recently returned from a long posting in Chicago, has been frustrated that his boss at the F.B.I. office, a white man named Roy K. Moore (an actual historical personage, here played by David Strathairn), keeps him and the office’s two other Black agents from working on civil-rights-related cases. So he accepts this top-secret mission with at least a modicum of enthusiasm. Having been a friend of the murdered man, he’s especially motivated to gather information about the killers, but he’s dubious about Gregory’s methods, which he witnesses up close on the first day of their partnership.

Wayne picks Gregory up at a motel, in Mobile, Alabama, and hands him a briefcase containing, among other things, a gun. As the pair drive by night to Hattiesburg, they’re stopped on a lonely country road by a white police officer. When Wayne presents his F.B.I. badge, the officer, either incredulous or enraged, accuses him of impersonating a federal official, slams him onto the hood of the car, and is about to shoot him at point-blank range when Gregory guns down the cop; to make extra sure, he shoots him again on the ground. Wayne is aghast—he is, after all, an officer of the law. “You shot a cop,” he exclaims. “We’re fucked.” Gregory responds, “No, he’s fucked.” I won’t spoil the upshot; suffice it to say that Wayne had no idea what Gregory had been up to in the area in 1964 and that Gregory fills him in on the backstory.

The prime dramatic mechanism of “By Any Means” is Wayne’s conversion—first grudging, then fervent—to Gregory’s methods. To start with, knowing the lay of the land, he guides their investigation but hesitates to set foot in segregated spaces, such as a gun shop where a weapon used against Vernon was sold. And he vehemently refuses to take part in the violence that’s Gregory’s method. But the physical threats ramp up. Attacked by whites in the street and facing violence at the house where he lives with his wife, Allison (Nicole Beharie), Wayne makes common cause with Gregory. Although there’s a practical and personal basis to Wayne’s newfound commitment, there’s also a philosophical one that’s reflected in the movie’s title. Wayne is effectively caught between the nonviolence advocated by Martin Luther King, Jr. (whose book “Why We Can’t Wait” he keeps in his car) and the teaching of Malcolm X, who made “by any means necessary” an iconic phrase of the civil-rights movement in a 1964 speech. Gregory says that he likes Malcolm—and quotes that very phrase. Wayne says that he wants to “put those crackers in jail” and asks Gregory what he’d do with them if he were Black. “Kill them all, every last one,” the gangster answers. Neither Wayne nor, apparently, anyone else he knows in Hattiesburg has entertained any such plan; nonetheless, the expression of it, however flip, suggests a counterfactual political fantasy that would make “One Battle After Another” seem downright passive.

There’s a tendency all too common in crime dramas that I’m tempted to call convenient incompetence: a major crime is committed, but police, media, and even lay people fail to notice much about it, handily allowing the plot to proceed without the complex interference of things that would certainly occur in the real world. In a way, it feels churlish to pick at “By Any Means” for this, because, at its best moments, it honors the exalted struggles that it dramatizes. This is the director Bratton’s second feature; his first, “The Inspection,” from 2022, based loosely on his own experience as a Black and gay marine in the age of “don’t ask, don’t tell,” is rich in such spiritual energy even as it looks analytically at the institution of the military in American civic and domestic life. The best moments in “By Any Means” build on that achievement. When Wayne and his wife attend Vernon’s voter-registration rally and a subsequent gathering at his home, she tells Vernon and his wife, Ellie (LaChanze), of Wayne’s frequently repeated line that “America won’t be right until Mississippi is right.” And there’s a fine scene in which Gregory, assuming that Wayne is the first Black F.B.I. agent, calls him the agency’s Jackie Robinson; Wayne responds with the names of a number of early Black agents, including the one believed to be the first, James Wormley Jones, who joined in 1919.

The movie’s very premise, the F.B.I.’s extreme effort to find Vernon’s killers, is just one of several elements in the film that may produce a kind of agonizing nostalgia. Despite the manifest horrors it confronts, “By Any Means” depicts a functioning federal government that has passed civil-rights legislation, enforced it (yes, even by any means necessary), and doesn’t have to worry about bad-faith Supreme Court decisions negating or neutering it. There is a President, Lyndon B. Johnson, who has no intention of letting vigilantes and racist local law enforcement “make a mockery” of the civil-rights legislation that he’s got through Congress. Even the plain and factual reporting of news on the radio seems like a vanished trace of an ancient society.

Still, the movie does at times rely on convenient incompetence. When white people get beaten up or injured, as gravely as by torture or as dramatically as in a car chase, the police appear to be unaware or indifferent, as do Black and white townspeople alike. A dead white police officer sparks no apparent interest, outcry, or investigation. There’s a noteworthy scene, sickening but instructive, in which a local Klan leader speaks to a handful of fellow-marauders and lays out a litany of grievances against Black people rooted in historical myth, economic anxiety, and class resentment, railing at the prosperity of one Black man rather than at the wealth of many white grandees. But the Black community’s politics—for that matter, politics in any practical sense—is entirely elided.

Above all, the script, by Sascha Penn, doesn’t give Wayne much interiority and offers little chance for him to express himself. The action takes place in a vacuum, sealed off from inner lives as from wider currents. In place of a view of history, “By Any Means” puts forth a Machiavellian world view: as the face of policy, nonviolent protest and law-abiding law enforcement are best—but, in practice, governments and activists alike will only succeed through ruthless, albeit covert, violence.

That’s why the movie’s many powerful moments are undercut by occasional, discordant touches that bring to mind the misjudgments of “Green Book” (2018), in which, similarly, a slickly rugged Italian New Yorker travels by car with a Black man down South and schools him in the cruder side of life (which that awful movie portrays as the authentic Blackness that its Black protagonist is missing). There’s no such animus in “By Any Means,” but the changes in attitude toward the strategic use of violence that Wayne undergoes as a result of his relationship with Gregory, are something he doubtless reflects on. He’s warned by his superiors to keep the mission a secret. Does he nonetheless talk with his wife, or with a friend? If not, is the secrecy of his hard mission weighing on him? And who is the person on whom it would be weighing, the person behind the badge, behind his official responsibility and self-designated purpose? “By Any Means” is often stirring but often infuriating. In shaping it as an action movie, Bratton makes it a tale of types, not of lives led but of pieces on the chessboard of history. ♦