Feuding families vie for water rights in the old West.

Upon arriving in the small Western town of San Rafael, Maryland shipping magnate and former sea captain Jim McKay is met by his fiancée, the vibrant Patricia Terrill, daughter of cattle baron Maj. Henry Terrill. The foreman of Terrill's Ladder Ranch, Steve Leech, accompanies Pat and mildly pokes fun of Jim's formal Eastern attire, but Jim is not offended. Pat then drives Jim out of town to Ladder, but stops at the ranch of her best friend, schoolteacher Julie Maragon, to introduce her to Jim. On the long ride to the ranch, Pat and Jim are harassed by the Ladder's neighbor, Buck Hannassey, and some of his Blanco Canyon ranchhands. When Buck playfully shoots off Jim's bowler hat, Pat pulls her rifle, but Jim prevents her from firing. Although Jim is then roped and pulled off the wagon by Buck and his men, Jim refuses to resist and, believing that he is a coward, Buck and the men soon withdraw. Pat expresses amazement that Jim prevented her from using her gun and his response is delight at finding his hat unscathed. Returning home, Buck visits Julie to suggest that he and his father would like to join their land with her ranch, the Big Muddy, named after the valuable river that runs through it. Julie declines Buck's blunt offer. The next morning, Jim refuses Steve's attempt to goad him into riding the ranch's wild stallion, Old Thunder, disappointing the ranchhands. At breakfast, Jim presents Henry with a gift of dueling pistols that belonged to his father, but when Jim expresses a distaste for violence, Henry advises him that violence is the way of life in the West. After admitting what happened with the Hannasseys, Jim is taken aback by Henry's fury and refuses to participate or sanction Henry and Steve's plan for a retaliatory raid. Overhearing the discussion, Pat is puzzled, then irritated by what she assumes is Jim's cowardly behavior. Later, while Henry and the Ladder men wreck havoc on the Blanco Canyon ranch, Jim remains with long-time ranchhand Ramon Gutierrez and gains his respect as he struggles to ride Old Thunder. Meanwhile, Henry, Steve and the Ladder men trap three of Buck's men in town while Buck hides in a nearby trough as his men are tortured in front of the townspeople. That evening at the Terrill's party to announce Pat and Jim's engagement, Henry assures Jim that the raid on the Hannasseys was necessary. Pat continues to sulk over Jim's behavior, but reluctantly joins the party. The festivities are interrupted by Buck's father Rufus, who breaks in to berate Henry for the raid and persistent antagonism against him, which disturbs Jim. The following day, Buck pleases Rufus by confiding that he believes that Julie will accept his marriage proposal. Late that afternoon, Jim, disturbed by the rivalry between the Terrills and the Hannasseys, departs Ladder for an overnight camping trip on the range. Learning of Jim's plans from Ramon, the Terrills and Steve are certain that Jim will get lost, and so go in search of him. Before leaving, Steve assails Pat for wanting to marry such a weak man. Although Pat tells Steve off for insulting Jim, in private she weeps in bitter confusion over Jim's behavior. Out on the range, Jim passes the night without incident and in the morning rides onto the Big Muddy, surprising Julie. After showing Jim around, Julie relates that her ranch is the area's sole water source and was deeded to her grandfather by the King of Spain. Julie admits that she wants to sell the property, but is dedicated to maintaining her grandfather's policy of allowing both the Terrills and Hannasseys access to the water, knowing that should she sell to either family, they would ruin the other. Jim then offers to buy the Big Muddy, promising to continue Julie's peace-keeping plan. Later, when Steve returns to the house after an unsuccessful search for Jim, a worried Pat encourages Henry to begin a new search. That evening, Jim rides into Henry and Steve's range camp and once back home insists that he was never lost. Steve calls Jim a liar and challenges him to a fight in front of Henry, Pat and several hands, but Jim refuses to be baited into an argument. In private, Pat and Jim have a bitter argument when she demands to know why he continues to humiliate her by his cowardly behavior. When Jim asserts that he has nothing to prove, and offers to move into town, Pat agrees that they should rethink their engagement. Just before dawn, Jim awakens Steve and the men go far from the main house for a drawn-out fistfight. Afterward, Jim confounds Steve by commenting on the futility of their act. That afternoon Steve leads the Terrill men on a drive to force the Hannessey cattle away from drinking at the Big Muddy. When Rufus learns that several of his cattle have died due to lack of water, he orders Buck to bring Julie to Blanco forcibly to make a decision on Big Muddy. Meanwhile, when Julie visits Ladder and learns that Jim has moved into town, she confronts Pat, who angrily insists that Jim has behaved incomprehensively. Julie is incredulous, and insists that if Pat loved Jim she could never believe him a coward. When Pat declares that Jim's behavior has shamed her, Julie reveals that Jim purchased the Big Muddy to give to Pat as a wedding present. Upon returning to the Big Muddy, Julie finds Jim and encourages him to return to Pat. Jim maintains that the relationship is irreparably broken, but that he intends to remain to work the Big Muddy. Just after Jim departs for town, Buck and his men arrive to take Julie to Blanco Canyon. Pat then meets Jim at his hotel, hoping to make amends, observing that Henry will be pleased to join the Big Muddy to Ladder. When Jim informs Pat that he will manage the Big Muddy his own way, however, she is outraged and walks out. At Blanco Canyon, Rufus tells Julie that he will hold her captive to lure Henry to the ranch to settle accounts. Rufus then suggests Julie can prevent the conflict by agreeing to marry Buck, but Julie is appalled, then reveals that she has sold the Big Muddy to Jim. That night, Buck attempts to force himself on Julie, but she is rescued by Rufus. When Buck turns on Rufus, he manages to fight off his son, but warns him that one day he will have to kill him. Meanwhile back at Ladder, Henry and Steve learn of Julie's kidnapping and set off for Blanco Canyon. Alarmed, Ramon tells Jim, who intercepts the men, insisting that he can rescue Julie without resorting to violence. The men ride to the Canyon the next morning, but allow Jim to meet the Hannasseys alone. When Jim approaches, Buck tells Julie that he will kill Jim if she departs with him. When Julie implores Jim to leave, Rufus realizes that she is lying and that she cares for Jim. Rufus then intercedes when Buck attacks Jim, who is unarmed. Meanwhile, in the canyon below, Steve refuses Henry's order to storm Blanco. When Henry determinedly sets off toward the Hannasseys' alone, however, Steve and the other men eventually follow. At Blanco, Rufus is disgusted by Buck's behavior and forces him to participate in a formal duel with Jim. During the duel, Buck cheats and shoots early, but only grazes Jim. Rufus is further humiliated when Buck then cowers in fear of Jim's clear return shot. After Jim fires into the ground and turns to escort Julie away, Buck tries to shoot Jim in the back, but Rufus kills him. When Henry leads the men unknowingly into an ambush by the Hannassey men, Steve is wounded and a number of men killed. As the fighting intensifies, Jim leads Julie away, but Rufus stops him to declare that the battle should be only between him and Henry. When Jim informs Henry, he agrees to the challenge. With the Ladder and Hannassey men watching, Henry and Rufus square off in the canyon and kill one another. Steve weeps bitterly over the futility of the fight as Jim, Julie and Ramon ride off together.