There's a difference between "long" and "epic," although in movie terms the two frequently get confused. Martin Scorsese delivers the former but not the latter with "Killers of the Flower Moon," his second consecutive three-and-a-half-hour movie derived from a true story and underwritten by a prestige-hungry streaming service.

In this case, it's Apple TV+, after Netflix let the veteran director run free in the editing room with "The Irishman." The result turns out to be similar in its weaknesses and less pronounced in its strengths, yielding a stark, even bleak, look at the wanton murder of Native Americans to obtain their oil money a century ago, while local authorities turned a blind eye.

Painfully earnest, and committed to both historical and cultural accuracy, Scorsese has teamed with friends old and newer, starting with his decades-long collaborator Robert De Niro, who joins him in what feels like a valedictory dance exposing America's violent history. "Flower Moon" also marks Scorsese's sixth feature-length film with Leonardo DiCaprio, who, partly due to the limited nature of his character, represents the weak link in giving the movie the emotional depth or heft that would match its scope and scale.

The real star in that sense is Lily Gladstone as Mollie Kyle, the Osage woman whose family is among the heirs to those oil riches, which made them the wealthiest people on Earth per capita while paying an almost unimaginable price for it. It's a strong, natural performance, hindered by gaps in the character as the script tilts toward Ernest.

Despite passing himself off as a friend to the Osage Nation, the town's leading citizen, Bill Hale (De Niro), has set his sights on gaining control of those oil rights, seeing his nephew, DiCaprio's Ernest Burkhart, as another means of achieving that. Newly returned from World War I, Ernest arrives in Oklahoma eager to make money, with marriage to an Osage woman offering the quickest path to wealth -- a dynamic that both sides of the equation clearly recognize.

Nevertheless, the sober Mollie inexplicably can't resist Ernest, who begins by driving her around in his cab and gradually sets about wooing her. Complicating the grim business at hand, the simple Ernest genuinely grows to love her, even as he participates in his uncle's increasingly ruthless scheming to eliminate anyone who might stand between him and all that cash.

As constructed by Scorsese, who shares screenplay credit with Eric Roth in adapting David Grann's book, the film takes its sweet time chronicling this cascading series of events, which for many will serve as a revelatory chapter in US history. A reference to the 1921 Tulsa Race Massacre provides useful context, speaking to brutality visited upon people of color a century ago with little fear of punishment or reprisal.

The possibility of that finally comes roughly two thirds of the way through the film with a visit from the nascent FBI (they introduce themselves as the Bureau of Investigation) and an agent portrayed by Jesse Plemons. The narrative shifts gears at that point, in a helpful way that makes one wish they had introduced him much sooner.

After all the great movies Scorsese has directed, his streaming phase has birthed better-than-average ones, perhaps because the finished product is in some respects secondary to the reputational value associated with the director and the stars he attracts.

Notably, "Killers of the Flower Moon" will receive a wide big-screen launch that includes playing on hundreds of Imax screens. Like another long and weighty film, "Oppenheimer," its qualities as a handsome period drama don't demand the format but they do feed a sense of bigness, which was surely the goal.

"Oppenheimer" certainly demonstrated audiences would sit through such an experience, but that still feels like an exception. Either way, Scorsese has served up a movie that also plunges into a dark history and comes away with less bang for its bucks.

"Killers of the Flower Moon" premieres October 20 in US theaters and will later play on Apple TV+. It's rated R. (Disclosure: Lowry's wife works for a unit of Apple.)