The storyline is pretty flat for a Western, but the cast and their deliveries make this quite worthy viewing.
McQueen's parents were killed by three dirty cowpokes and he doggedly hunts them down for revenge. He nails the first two, but his travels along the way -- a couple of pious women, a supportive friar -- have taught him that perhaps forgiveness is the better route, so when he corners that third killer ... well, you get the picture.
Still there's a lot of MidCentury acting power on deck here and they are what make this film work. McQueen, of course, but also very good work from Karl Mauldin, Arthur Kennedy, Suzanne Pleshette, Janet Margolin, Pat Hingle & Martin Landau. It's really the string of these performances, not the plot, that keep this film moving along.
According to IMDb, Loni Anderson is an uncredited saloon gal; I never noticed her. There WAS another saloon girl with a short but interesting delivery, the saloon girl who was Martin Landau's wife. When McQueen turns up in her hotel room, she portrays well a woman who's extremely titillated to be in a bedroom with the man who planted her husband in the ground. That's Joanna Moore, who later wed Ryan O'Neal and mothered Tatum O'Neal.
Numerous sources claim this 1966 movie is a prequel to Harold Robbins' 1961 novel The Carpetbaggers. Not exactly true. The novel does contain a Nevada Smith character, and this film fleshes out some of his background as briefly presented in that novel, but that's about it. The Carpetbaggers is loosely based on the early life of Howard Hughes. And Hughes' 1930ish Hollywood adventures have nothing to do whatsoever with the 1880ish Texas in which this film is set.
And this film's screenplay was from the hand of John Michael Hayes, with no meaningful involvement of Robbins at all. The "real connection" was likely nothing more than a fee paid to Robbins in a half-hearted attempt to capitalize on The Carpetbaggers' sexually-steamy success (which was on par with Lady Chatterley's Lover and Tropic of Cancer in its day). That done at a time when viewer interest in the Western genre was fading fast into the sunset.
RECOMMENDATION: If you a Western fan and you've missed it, you need to fill in the gap.