Leonardo DiCaprio, left, and Lily Gladstone in “Killers of the Flower Moon.” Melinda Sue Gordon/Apple TV Plus

The four most dreaded words for a film critic are, “What did you think?” And never have they been more problematic than when it comes to “Killers of the Flower Moon,” Martin Scorsese’s eagerly anticipated adaptation of David Grann’s 2017 book of the same name.

In that gripping, magisterial account, Grann chronicled in sickening detail how a group of Osage Indians in 1920s Oklahoma were exploited, terrorized and murdered in a series of mysterious crimes. It wasn’t a complete surprise that the culprits turned out to be the white neighbors – politicians, businessmen, friends and even loved ones – who pretended to be the Osages’ allies and protectors. Although the literal crime would eventually be solved by agents of a nascent organization called the Bureau of Investigation (later known as the FBI), what propelled “Killers of the Flower Moon” was Grann’s carefully calibrated way of widening the scope of the malfeasance, as what seemed initially to be a lively, pluralistic boom town morphed into a microcosm of American capitalistic expansion at its most ruthless, rapacious and racist.

Scorsese, working from a script he co-wrote with Eric Roth, does away with the suspense Grann generated so expertly in his book: After a prologue depicting a Native American funeral ritual, and a newsreel-like introduction explaining the vast oil reserves that made the Osage the wealthiest people in the country, he gets the narrative underway on a train carrying recent World War I veteran Ernest Burkhart (Leonardo DiCaprio) to Fairfax, Oklahoma, where he intends to seek his fortune under the guidance of his wildly successful uncle, Bill “King” Hale (Robert De Niro). Hale effectively sets up the scheme within the first 20 minutes of “Killers of the Flower Moon,” explaining to the none-too-bright Ernest that the Osage are “the finest, most beautiful people on God’s Earth” before adding that there’s money to be made in laying claim to the Indians’ rights to the oil under their tribal lands – by way of marriage, murder or any means necessary.

Robert De Niro, left, and Jesse Plemons in “Killers of the Flower Moon.” Melinda Sue Gordon/Apple TV Plus

Scorsese’s choice to lay out the plan so bluntly deprives “Killers of the Flower Moon” of the crucial element of suspense: By the time the Bureau of Investigation’s Tom White (Jesse Plemons) shows up two hours in, the audience knows full well whodunit (as Scorsese has repeated several times in interviews, this story is a who-didn’t-do-it). What we’re left with is a dreadful, sometimes surpassingly dull taxonomy of wickedness, as the greedy, lunkheaded Ernest succumbs to Hale’s venal spell, while also falling in love with and marrying an Osage woman named Mollie.

Played with serene knowingness by Lily Gladstone, Mollie is the moral conscience of “Killers of the Flower Moon.” But she’s mostly a victim, meaning that she’s often relegated to a role of passive, if bitterly affecting, suffering. The doers here are the bad guys, much like in Scorsese pictures past, but now their impunity isn’t a matter of escapist wish fulfillment and scoundrel-y derring-do. Instead, it possesses what it’s probably had all along: the petty, plodding rhythms that befit evil at its most banal. With his mouth drawn down into a marionette frown, DiCaprio delivers one of his mumble-mouthed, anti-charismatic portrayals (more “The Revenant” than “The Wolf of Wall Street”), while De Niro embodies Hale like a down-home version of one of his New York goombahs. Scorsese lards the supporting cast with musicians like Jason Isbell and Jack White; by far the most impressive is Sturgill Simpson, who provides a welcome gleam of sly humor as one of Hale’s moonshining henchmen. (The musical score, by the late Robbie Robertson, consists mostly of a brooding bass line ostinato.)

There’s no doubt that “Killers of the Flower Moon” reflects a shift in energy that is defensible – even necessary – from an ethical point of view. Narratively, that pivot results in a film that, it must be said, feels leeched of the energy and vigor viewers associate with Scorsese at his most exhilarating. In recent years, with films like “Silence” and “The Irishman,” fans have been forced to adjust their metabolisms and tamp their hunger for vicarious thrills. Like those films, “Killers of the Flower Moon” is a slower, more methodical, sometimes more boring affair. To be sure, the broad contours align with Scorsese’s most famous crime pictures: There are moments when Hale’s plans resemble the heists and hits of “Goodfellas,” and there are even a couple of shot-for-shot echoes. But here, the villainy is muted, as dirtified and desaturated as cinematographer Rodrigo Prieto’s color palette. As the brazenness and bodies pile up, the scams are no longer flights of hubristic fancy; they’re chores to be endured. (No Copacabana tracking shots or “Layla” piano solos here.)

Robert De Niro, left, and Leonardo DiCaprio in “Killers of the Flower Moon.” Melinda Sue Gordon/Apple TV Plus

If “Killers of the Flower Moon” isn’t as purely pleasurable to watch as Scorsese’s most canonical movies, that doesn’t mean it lacks beauty, or even audacity. Some of the film’s most transcendent moments capture the swirl of life in Osage County, from its weddings to its family meals; many feature Mollie’s mother, Lizzie (Tantoo Cardinal), whose experiences on the brink of death are represented in stunning flights of magical realism. The chaotic town of Fairfax, where people ride on horses and racecars down the main street, is a fascinating jumble of Old West and modernity, its veneer of optimism and progress queasily coexisting with the Ku Klux Klan and White-led race riots in Tulsa, just 65 miles away. As in the book, the subtext of “Killers of the Flower Moon” is what might have been, as a brief dream of tolerance and coexistence curdles into an engulfing exercise in cultural and financial theft.

As a work of history and heightened political consciousness, “Killers of the Flower Moon” is beyond reproach; it dramatizes a grievous truth – about the depravity, destruction and self-deception that undergird the American idea – that has been buried for too long, especially in movies. But that nobility of purposes raises uncomfortable questions about what makes for riveting cinema – or at least a riveting Martin Scorsese movie. At 3 1/2 hours, the movie tests the audience’s tolerance for episodic rehearsals of bad deeds done; by the time we get to the inevitable courtroom drama (featuring a distractingly cast Brendan Fraser), the proceedings feel rote and anticlimactic.

In interviews, Scorsese has explained how he and Roth rewrote Roth’s original script for “Killers of the Flower Moon,” to give the Osage more space but also to tell their story from the inside. Despite those efforts, his point of view never gets deeper than that of an alert, caring observer. That’s despite an obvious emotional attachment to Mollie, a connection that becomes apparent in the film’s epilogue, in which the director creates a set piece that feels both emotionally distancing and movingly on point. It’s startling, self-conscious and strangely of a piece with the admirable, vexingly uneven movie that has come before: In other words, it’s totally Scorsese.