Tall in the Saddle begins in the late 1800s. A gritty cowboy named Rocklin (John Wayne) boards a stagecoach bound for Santa Inez, Arizona. Driving the stagecoach is Dave, a frontiersman spewing authentic gibberish about all the goings on in the area. Dave especially gets a kick out of sending his two female passengers, Miss Elizabeth Martin and her niece Clara Cardell, on a rough ride through the mountainside.
They stop at a roadside inn to rest their horses, and meet Sheriff Jackson and Bob Clews. The two men there to investigate some incidents involving cattle rustling in the area. Rocklin’s in town to see Red Cardell who owns the K.C. Ranch. Some of his cattle were also stolen. Come to find out, someone recently murdered Red, and Clara’s great is his niece.
Cantankerous old Dave drunkenly throws some insults at the two men, so they take him to the barn to “lie down”. Rocklin later finds him unconscious from a pistol whipping, so Rocklin drives the stagecoach the rest of the way to town.
Rocklin checks into his hotel where the town’s lawyer, Robert Garvey, invites him to play poker with him and the boys. When Clint Harolday, the prick stepson of the owner of the Topaz Ranch, tries to cheat a hand, Rocklin stops him, and claims the pot for his own.
Clint pulls a gun on Rocklin, and Rocklin calmly walks upstairs to his room. He quickly returns with guns of his own to take what he rightfully won. The next day, Clint’s sister Arly demands Rocklin hand over her brother’s winnings. He ignores her, so she fires her gun at him. Rocklin walks away without escalating the situation.
When Rocklin learns Garvey was Red’s lawyer, Rocklin gives him a handwritten letter from Red hiring him as foreman of the K.C. Ranch. Miss Martin and Clara enter Garvey’s office too where Miss Martin declares that the K.C. Ranch is now Clara’s.
Miss Martin’s lack of tact embarrasses Clara, and she insists Rocklin keep the $150 payment her great uncle advanced him for coming to town. Rocklin politely declines of course. Next, Arly pops in for a visit, and tells Rocklin that her stepfather now wants him as the foreman of his Topaz Ranch. Rocklin accepts, but when he meets Harolday, Arly’s stepfather, he sends Rocklin to a deserted camp at the edge of his ranch to look for the cattle rustlers who Harolday believes killed Red.
Once at the camp, Dave delivers Rocklin a letter from Clara which she returns the $150, and tells him that she can not trust Garvey as he convinced her aunt that Clara should sign the ranch over to him, and return home back east.
Dave tells Rocklin that Red and Garvey were gambling buddies, and Red always lost. On the day Red was murdered, he was headed to visit the district judge to present evidence that Garvey was using marked cards. This leads Rocklin to conclude that Garvey is a card cheat and murderer.
By now, Arly has learned that Dave delivered a letter to Rocklin from Clara. So, she goes to visit him. As she nears Rocklin’s cabin, someone tries to shoot him. He goes outside to look for clues. When he goes back inside, he finds Arly looking through his belongings where she spots Clara’s letter. Jealous, she has a tantrum and rips up the letter. She then fires him, and orders him off the ranch. She’s so angry, she throws her knife at him; nearly missing. Rocklin grabs her in his arms, gives her a hearty kiss on the lips, and then walks out.
Back in town, Rocklin tells Harolday and Clint about the shot taken at him, and he shows Clint a leather pouch he discovered outside the cabin right after the shots. While Clint claims he never saw the pouch, once Rocklin leaves, Harolday tells Clint he knows he lied, and he needs to leave town.
Clara visits Rocklin back at his hotel where she tells him Miss Martin is going to sign an affidavit saying Clara is underage which would revert control of the K.C. Ranch to her. However, Clara already provided Garvey a letter affirming her legal age.
Rocklin heads over to Garvey’s office to find that letter, but Garvey already burned it. He does find two decks of marked cards in Garvey’s desk, and he accuses Garvey of killing Red. Rocklin says he’s gonna show the cards to the district judge, so Garvey pulls his gun on Rocklin. The two start brawling, and Garvey winds up unconscious.
Looking to start some trouble of her own, Arly is at the hotel, and she lets it be known that she and Rocklin made out, and he like likes her. This upsets Clara, but Rocklin lets Arly know in no uncertain terms that no woman will ever hogtie and brand him. No siree, bub!
Depressed, Clara returns back to her ranch. Her stuffy aunt gets her to admit that she’s going to head over to the district judge with Rocklin to have Garvey investigated. So Miss Martin sends a letter to Garvey warning him. That night, Dave abducts Clint, and brings him to the hotel so Rocklin can question him about Garvey’s involvement in Red’s murder.
Outside the hotel, Arly’s handyman, Tala, spies on the place. He witnesses someone climb outside the stairs of Rocklin’s room. From inside, Clint spots someone reach through the window, and snatch Rocklin’s gun. The person aims it at Rocklin, and Clint shoves Rocklin out of the way. Clint falls to the ground dead as the mysterious person tosses Rocklin’s gun to the floor, and runs off.
When the townsfolk here the commotion, they rush to the room where they find Rocklin with the smoking gun in his hand; standing over Clint. Garvey accuses Rocklin of killing Clint, but he denies it, and jumps out the window.
Garvey rounds up a posse, and they chase Rocklin to the K.C. Ranch. Tala tells Arly that he witnessed who killed her brother, and it wasn’t Rocklin, the two take a shortcut to the ranch before the posse can get there.
Back at the ranch, Miss Martin admits to Clara that Rocklin is actually Red’s nephew. He is the one who will inherit the ranch, and she and Garvey tried to stop that from happening. Rocklin and Dave hear the whole confession, so ornery old Dave ties her up to get her out of the way. When Bob and George Clews show up, they disarm Rocklin, and knock him out. Miss Martin has the brothers take him back to town.
The posse heads out, while Miss Martin and Garvey discuss their next moves. Unbeknownst to them, Rocklin is in the room next door; eavesdropping on them. Harolday rides up to the ranch, and as he prepares to shoot Rocklin through the window, Arly and Tala stop him. Arly lets Rocklin know that Tala saw Harolday kill Clint, and the mysterious pouch belongs to Harolday too. Rocklin’s no dummy. He realizes that is was Harolday who murdered his Uncle Red. Harolday tries to escape, so Tala kills him with his knife.
Garvey now confesses that Harolday was the real brains behind the attempt to acquire all the ranches in the area. He wanted to split them up in to tiny plots, and sell them to farmers. Garvey says Harolday only killed Red because he threatened to expose him as a card cheat. Also, the bullet that killed Clint really was meant for Rocklin.
With the murder/mystery solved, Arly spots Rocklin and Clara getting chummy about their future, so she sadly leaves. However, Clara doesn’t belong in the west, and she tells Rocklin she’s going back east. She knows he belongs to Arly anyhow; so does Rocklin. The film ends with Dave and Tala watching Rocklin and Arly passionately kiss into the black and white sunset.