The first thing, but not the only thing, to say about “Killers of the Flower Moon” is that it is long. At nearly three and a half hours, it is a lot of time in front of the silver screen during the age of fragmented screen time. It makes “Oppenheimer,” itself three hours, look like a Super Bowl commercial. It is also ambitious, an American film about America by an American master, Martin Scorsese. It is not without pleasure. Its purpose, though, is another matter. It is a tale of original sin, jauntily told. 

“Killers” takes its cues from a book, a best seller, by the same name by a journalist with a Midas movie touch, David Grann. The movie’s MacGuffin makes an early appearance, with a slow-motion geyser of black gold erupting from the land the Osage tribe bought after a long sojourn to Oklahoma from Ohio. The oil looks like a boon, but it will soon prove to be a burden. As the Osage people dance through its spurt, it can almost be mistaken for blood.

That different liquid soon appears in buckets, largely due to William “King” Hale, played by Robert De Niro, and his nephew, Ernest Burkhart, played by Leonardo DiCaprio. Hale is a figure out of William Faulkner or Cormac McCarthy, a monster in a fedora who drinks whiskey by the fire and serves as judge, jury, and executioner for the Osage with whom he feigns friendship. Ernest is a drifter who washes up in Osage country and proves a useful pawn.

The murderous scheme they hatch is, in the first instance, a legal one. This true-crime Western tells the story of dozens of murderers of Osage in 1921. The setting is Fairfax, which Mr. Scorsese and the screenwriter Eric Roth resurrect as a boomtown on the make. A headline from that year in the New York Times reads, “Osage Are Richest People: Greatest Per Capita Wealth in World Results From Oil Deal.” It was to be a prelude to misery.

At the crux of the coming cruelty were 2,229 Osage headrights, one for each member of the tribe in 1906. These are property rights, entitling a holder to a quarterly payment from the Osage Mineral Estate — oil royalties. Between 1918 and 1931, more than 60 Osage were murdered in what is called the “Reign of Terror,” slain for their share in the underground treasure. “Killers” zeroes in on white men who married and then murdered their Osage wives. At Osage country, matrimony was a species of conspiracy.   

Mr. DiCaprio’s Burkhart is at first a driver to Lily Gladstone’s Mollie Kyle, an Osage woman, before taxiing into her heart. Their relationship gives “Killers” an idiosyncratic heart. Kyle knows Burkhart wants a share of her fortune, but this does not disqualify his love — it throws it into relief. Still, Burkhart’s ultimate loyalty is to his uncle, and together they embark on a plan to, in a stepwise fashion, eliminate Kyle’s family to lay hands on their headrights.

The means employed toward this end are as varied as those enumerated by the singer Leonard Cohen in “Who By Fire.” One family’s house explodes. There are lots of bullet holes and staged “accidents.” Kyle herself is slowly poisoned, her diabetes used as an excuse for a cocktail of drugs meant to keep her in a haze. Despite the wealth of the Osage, the law saw them as “incompetents” in need of guardians. Here, custodianship takes on a sinister hue. 

The plot, or at least the part of it with which “Killers” is concerned, is unraveled by an unlikely source, then as now  — the federal government. The subtitle of Mr. Grann’s book is “The Osage Murders and the Birth of the FBI,” and the film’s latter portion is devoted to how federal law enforcement, still in its embryonic phases, unraveled King Hale’s web of extortion and murder. The first director of the FBI, J. Edgar Hoover, is offhandedly mentioned.

The Bureau, mired in years of controversy that have seen it transmogrified into a political lightning rod, will likely appreciate the positive press. The nattily dressed gentlemen investigators crack the case and deliver a measure of justice to the Osage. The whole tale can feel lacking in ambiguity, a kind of morality play with the parts assigned in advance, an expiation of guilt by a clear judging of the guilty and the innocent. 

There is no avoiding the fact that the history with which Mr. Scorsese works is sordid. The movie works better, though, less as a cinematic version of the “1619 Project” than as a kind of 19th-century “Goodfellas.” The director is especially good with close-ups and zoom-outs, adept at mapping a quivering lip or blinking eye as an oil rig or reddening horizon. One can almost discern Messrs. De Niro, DiCaprio, and Scorsese digging not for oil, but for gold statuettes.