It's an adaptation of the nonfiction book about the murders of dozens of Osage Indians back in the 1920s. The land the Osage ended up on turned out to be rich with oil, and greed lead to a conspiracy of murder to obtain their head rights to the oil. It was one of the first big cases tackled by J Edgar Hoover's Bureau that led to the rise of the FBI. I actually just finished reading the book yesterday and can't wait for the movie.