The Drowning Pool

The Drowning Pool is a movie starring Paul Newman, Joanne Woodward, and Anthony Franciosa. Harper's a big-city PI, who travels to Louisiana to help an old girlfriend who's worried her husband will find out she's been cheating on him.
Harper is brought to Louisiana to investigate an attempted blackmail scheme. He soon finds out that it involves an old flame of his and her daughter. He eventually finds himself caught in a power struggle between the matriarch of the family and a greedy oil baron, who wants their property. Poor Harper! Things are not as straight-forward as they initially appeared.