The elders of the Osage Nation hold a solemn ceremony to bury a sacred pipe, reflecting on the painful assimilation of their descendants into White American culture. As they wander through their vibrant Oklahoma reservation during the stunning "flower moon" season, several members of the Osage tribe stumble upon an extraordinary discovery of oil erupting from the ground. This lucky find catapults the tribe into immense wealth, as they retain ownership of mineral rights, allowing them to share in prosperous oil-lease revenues. Yet, due to legal requirements, white court-appointed guardians are assigned to manage the finances of full and half-blood members, branding them as "incompetent."
Remarkably, the Osage County Native Americans achieve the highest per capita GDP globally.
In 1919, returning from the harrowing experience of World War I, Ernest Burkhart (played by Leonardo DiCaprio) moves in with his brother Byron (portrayed by Scott Shepherd) and their uncle, William King Hale (enacted by Robert De Niro), on Hale's sprawling ranch near Fairfax. Hale, often referred to as "King," serves dual roles as a cattle rancher and reserve deputy sheriff, presenting himself as a benevolent supporter of the Osage, using their language and showering them with gifts. However, he privately views the Native Americans as weak and sickly.
During Hale's time, numerous Osage people have met mysterious, untimely deaths, with investigations into these incidents never occurring. These deaths predominantly affect individuals who possess valuable head-rights. In an alarming turn, Ernest and Byron participate in an armed robbery targeting the Osage community. In pursuit of employment, Ernest encounters Mollie Kyle (played by Lily Gladstone), who holds oil head-rights through her family. Mollie struggles against Accountant Pitts Beatty (depicted by Gene Jones) to access her funds as her mother suffers from illness and she lacks a legal guardian.
Ernest becomes invested in understanding Osage culture and history, forging a romantic relationship with Mollie. The couple ultimately marries in a ceremony that beautifully blends Roman Catholic and Osage traditions, raising three children together as their love grows.
However, Hale sinisterly orchestrates contract killings targeting affluent Osage individuals, believing that if Mollie's family faces tragedy, Ernest will inherit additional head rights. Mollie's health is precarious; she is diabetic, and her mother's condition worsens. Tragedy strikes when Mollie's sister Minnie (portrayed by Jillian Dion) succumbs to an inexplicable illness, leading to the transfer of her head-rights to her husband, Bill Smith (played by Jason Isbell). Following Minna's death, Hale advises Ernest that the death of Bill would promote further inheritance for Mollie, thus for Ernest.
The dire circumstances continue when Hale instructs Byron to eliminate Mollie's other sibling, the rebellious Anna (played by Cara Jade Myers). Byron collaborates with Kelsie Morrison (enacted by Louis Cancelmi) to carry out Anna's murder, pulling off this heinous act beyond the watchful eyes of the Osage.
Growing increasingly alarmed, Lizzie and the Osage council levy blame against the local white residents, urging the tribe for retaliation. They pick Barney McBride (also portrayed by Scott Shepherd), a white oilman, to head to Washington, D.C., and petition for federal intervention in the increasing violence. Tragically, Barney becomes a victim himself, facing unknown attackers in the capital.
As feelings of unrest escalate, a newsreel covering the 1921 Tulsa race massacre, where white residents decimated a Native American community, instills even more dread among the Osage. In her final moments, Lizzie finds solace in the sight of her ancestors welcoming her into the afterlife.
Hale, determined to further his plans, instructs Ernest to eliminate Mollie's last sister and her husband. However, when the assassination attempt fails, Ernest unwittingly sets off a disastrous chain of events. In a desperate move, he negotiates with Blackie Thompson (depicted by Tommy Schultz), offering his red Buick as a bargaining chip for the killings of Reta and Bill. Unfortunately for Ernest, this plot unravels when Blackie is caught and spills the secret to the authorities.
Surrounded by a web of deceit, Hale manipulates Ernest into a punishment ritual in a Masonic temple, reflecting upon their increasingly murky morality. With Hale wielding significant political sway, he manages to silence potential investigations into the murders. However, the dynamic shifts when Mollie discreetly hires private detective William J. Burns (portrayed by Gary Basaraba). Yet, Ernest and Byron fend him off the reservation, ensuring that their twisted storyline remains obscured.
Hale continues his manipulation and even orders the murder of Mollie’s friend to ensure his twisted plan comes to fruition. As the horror unfolds, the situation worsens, leading to the death of Henry, which is disguised as a suicide.
Mollie's determination shines as she rallies her community and, despite her ailing health, journeys to Washington to implore President Calvin Coolidge for assistance. Hale, sensing the escalating attention on their crimes, devises a cruel plan to slow her progress by poisoning her insulin. The illness cripples Mollie, yet ironically, it also affects Ernest, who suffers from his own failed actions.
In a twist, the Bureau of Investigation dispatches agent Thomas Bruce White Sr. (played by Jesse Plemons) alongside his team to delve into the series of suspicious deaths. Encouraged by eyewitness testimonies linking Kelsie to Anna's death, law enforcement finally begins to connect the dots leading to Hale and Ernest.
In an attempt to safeguard their empire, Hale takes drastic measures to liquidate his holdings and orchestrates additional crimes, but lifelines continue to tighten around him. The threats and repercussions rise when Blackie bears witness to Hale’s evil plot and is incarcerated instead of killed, further implicating Ernest.
As the investigation closes in, twelfth-hour confessions spill secrets of an insidious legacy. Hale's efforts to eradicate anyone who knew about his malevolent activities ultimately fail when agents rescue Mollie from a fatal fate and uncover the extensive poisoning she endured.
Eventually, Ernest, in a moment of clarity, agrees to testify against his uncle, driven by the desire to protect his children, especially after tremendous family losses. Despite W.S. Hamilton (played by Brendan Fraser), Hale’s attorney, attempting to manipulate him into recanting, Ernest's testimony becomes pivotal.
The aftermath sees Hale and Ernest brought to justice, both sentenced to life, with public outcries demanding accountability from Osage. Mollie, forever affected by the ordeal, eventually relocates, wed to another man, passing away from diabetes at the tender age of 50 in 1937 without a mention of her family's tragedy in her obituary.
The film draws to a close, an aerial view panning over a contemporary Osage powwow dancing circle, a vivid testament to the enduring spirit of the Osage Nation, hanging on through the shadows of their storied past.
