Martin Scorsese’s latest period epic after such works as “Gangs of New York,” (2002), “Age of Innocence” (1993) and “The Aviator” (2004) should serve not only as a history lesson to many, but more importantly as a bloody smear of shame and, hopefully, an uneasy point of reflection. For those who were here first, “Killers of the Flower Moon” will likely be a sad reminder of what was and continues to be. 

Working from journalist David Grann’s 2017 real-life account with the additional tag “The Osage Murders and the Birth of the FBI, ” there’s much in the film that leverages Scorsese’s mean-streets, gangland roots and much that unfurls that, if not stated as nonfiction, would be hard to believe. Set on an Osage reservation post-World War I, “Killers of the Flower Moon” has the grand, neo-western feel of Michael Cimino’s “Heaven’s Gate” (1980) and even Sergio Leone’s “Once Upon a Time in the West” (1968) as Leonardo DiCaprio’s Ernest Burkhart, returning from the war (a cook, not a soldier, because he has a weak stomach) steps off a train in Fairfax, Oklahoma, and is picked up by a dapper member of the Osage tribe by the name of Charlie Whitehorn. Charlie has better duds than Ernest, and a shiny new car. As they drive up lush prairie hills lined with prime-specimen cattle and oil derricks, Ernest asks: “Whose land is this?” “Mine,” Charlie beams. From there, newsreel footage explain how the Osage, part of the forced Indian relocations of the mid-late 1800s, became the richest per capita community in the world because under a scrubby landscape once deemed of little value lay a limitless bounty of oil. Happy riches this is not; by government decree the Osage were deemed not capable to manage their money and were given guardians – white attorneys, bankers, trustees and the like – to look after and allocate their riches. You can only imagine how well that worked out. 

Grann’s focus isn’t so much the oil money but the associated Reign of Terror, several years in the 1920s when some 60 Osage were outright murdered, died under suspicious circumstances or went missing, with local law enforcement doing little more than nodding their head. (That number is considered conservative.) Indifference to the plight of native peoples ripples onward: according to the FBI, the number of missing and murdered indigenous women now is nearly three times higher than the next demographic segment, a stark point underscored in Taylor Sheridan’s “Wind River” (2017).

What goes on in Scorsese’s “Killers of the Flower Moon” (the name taken from the Osage expression for the wildflowers that bloom across the prairie under a full moon) is a slow unraveling – the movie is more than three and a half hours – of an insidious, systematic plot to bilk and bleed the Osage. Ernest, a lazy idealist at best and not the sharpest of young men, has come to Fairfax under the prospect of prosperity that he expects to find in the employment of his rich uncle William “King” Hale (Robert De Niro), a cattle baron and, as we first meet him, a self-proclaimed great friend of the Osage. That couldn’t be further from the truth.

One of Hale’s early maneuverings is to employ Ernest as chauffeur to a young Osage woman named Mollie (Lily Gladstone), coincidentally next in line to inherit the family’s oil-rights share as the health of her mother, the matriarch of the clan (Tantoo Cardinal), is on the wane. They don’t live past 50, Hale remarks, implying that diabetes is a plague among the Osage (the “wasting disease,” he calls it). That initial sentiment seems to be one of genuine concern and pity, but scenes later, it’s revealed that modern medicine is employed as a ruse; insulin laced with poison is being administered to the Osage. The town’s doctors are among those in on the scheme; even the wide-eyed, simple-minded Ernest clearly knows something’s askew as he injects an ailing Mollie, now his wife, with the solution.

Hale’s long game is to marry, leech and inherit; when that doesn’t work quickly enough, a bullet to the head’s just as good, because without an eyewitness – and there never is one – the case is put atop of pile of similar unsolved deaths to gather dust.

It’s a hard emotional watch, the parasitic gutting of a community from the inside out. Even more deviously treacherous is Hale attending meetings of the Osage elders in an advisory capacity as they assemble in an effort to suss out who is behind the deaths, not knowing the devil is in the room. Hale and Ernest, along with Ernest’s brother, Byron (Scott Shepherd), married to Mollie’s sister Anna (Cara Jade Myers, excellent as the boozy flapper who speaks her mind freely and pays for it), aren’t anything like Henry Hill (Ray Liotta) in “Goodfellas” (1990) or even Jordan Belfort in “The Wolf of Wall Street” (2013). Those guys were predators who bared their fangs publicly and bit into the necks of other predators; here, Hale and his crew cloak themselves in wool and go after the ewes and lambs while acting the bull ram that keeps the non-existent menace at bay.

To Scorsese’s credit, he doesn’t render the Osage as victims. Mollie is at her most capable when at her most physically weakened; Gladstone plays her with a wry, knowing and deep internal resolve, not only holding her own with the two Oscar winners but wresting several scenes from them. Several efforts to elicit outside aid get snuffed violently by Hale, though ultimately, entreaties to President Harding by Mollie result in the arrival of a team of undercover investigators led by a gentlemanly former Texas Ranger (Jesse Plemons, perfect in the part) from the Bureau of Investigation, soon to be the FBI and newly overseen by a young J. Edgar Hoover (once played by DiCaprio in a Clint Eastwood biopic). Even then, Hale’s teflon armor and ability to spin and control the narrative feels uncrackable. How it all plays out is noted by history and Scorsese and his co-writer Eric Roth (“Dune,” “Munich”), inventively playing fast and loose with the fourth wall and formal law proceedings. It’s an inspired wrap-up that sees Brendan Fraser channeling his room-commanding heavy in “No Sudden Move” (2021), an ageless John Lithgow and rocker Jack White in sharp small turns.

De Niro, who seems minted for the unenviable role, hasn’t been this good in years and DiCaprio, with something of a Brando-esque mouthpiece, manages to make Ernest understandable, if not marginally sympathetic in a Greek tragedy of avarice-and-wrongs-realized sort of way. The other apt, emotionally evocative accent is the era-embracing score by rocker Robbie Robertson, peppered with Native American influences and southern slide-guitar twangs reminiscent of Ry Cooder’s work in “Southern Comfort” (1981). Robertson, the former guitarist of the Woodstock-era folk band The Band, is no stranger to Scorsese; the group’s farewell concert was the subject of Scorsese’s great 1978 rock-doc “The Last Waltz”; more to the point, Robertson, whose father is Mohawk, grew up on the Six Nations Reservation in Canada. His contribution are the heart and soul embers that burn within each frame. 

• Landmark Kendall Square Cinema, 355 Binney St., Cambridge; Apple Cinemas Cambridge, 168 Alewife Brook Parkway, Cambridge Highlands near Alewife and Fresh Pond; and AMC Assembly Row 12, 395 Artisan Way, Assembly Square, Somerville.