Retired U.S. Marshal James Arness Matt Dillon, now a fur trapper, is attacked on a riverbank by a group trying to steal his rifle and furs. Dillon is stabbed in the back and drifts down the river in the canoe until he is found by a wagoner and taken to Dodge City, Kansas. Everyone recognizes the beloved former lawman and he is nursed back to health. After three weeks of recovery, Dillon awakens to Miss Kitty, Amanda Blake a former saloon owner who has returned to Dodge from New Orleans with news that her old flame Dillon was dying.
Meanwhile, in the territorial prison, after twelve years, Steve Forrest Will Mannon is lashed one last time before release. He vows revenge on the judge and the marshal, Dillon. Overhearing his plans, fellow convict Jake Flagg takes it upon himself to warn Marshal Dillon, an old friend. After Mannon is released, Flagg takes the prison warden hostage to escape and track down Mannon. Unfortunately the warden tries to break free from Flagg, who was about to let him go free, and is wounded in the process. Flagg has no choice but to leave the disabled warden as he will be shortly found by the pursuing prison guards. After Flagg leaves, Mannon arrives and shoots the warden, knowing Flagg will be blamed for the murder. Later, Mannon finds the judge fishing on a river and murders him too. He makes for Dodge to find the marshal.
In Dodge City an inexperienced young cavalry officer, Ken Olandt Lt. Dexter, meets Dillon at the saloon and announces a reward for the capture of escaped felon and murderer, Jake Flagg. Lt. Dexter intends to bring Flagg to justice without help from his cavalry troop. Dillon knows Flagg is no murderer and goes to find him before the foolhardy officer gets himself killed trying to capture Flagg.
Flagg arrives at an Indian camp and jobfully greets his wife, Tantoo Cardinal Little Doe, and daughter, Patrice Martinez Bright Water. After he leaves to find Dillon, bounty hunters, the same men who attacked Dillon on the riverbank, led by Digger McCloud, arrive and take Flagg's wife hostage. Later, in the ravaged camp several more bounty hunters hold Bright Water, but she is rescued by Lt. Dexter and Dillon.
Mannon arrives in Dodge City demanding to find the marshal. Mannon and Miss Kitty each have a series of flashbacks. Talking to Hannah, the barkeep, Kitty reveals that she didn't marry Dillon because she couldn't bear to have him wounded near death all of the time, and she left for New Orleans when Dillon retired. She then recalls how the gunslinger and killer Mannon abused her and was finally brought to justice by Marshal Dillon twelve years ago. Once again, Mannon takes Kitty captive to blackmail Dillon. The new marshal, Buck Taylor Newly rides to bring Dillon to Dodge.
Flagg reunites with his daughter, Bright Water, and Dillon, and they are about to rescue his wife when Lt. Dexter shows up and gets wounded the next day in the gunfight. Too slow to outrun the bounty hunters, Flagg sacrifices himself to hold off the pursuit while Dillon takes the women and the wounded officer to the trail to the fort. Dillon returns and fights off the bounty hunters, but Flagg is killed before he can go to Canada.
The new marshal Newly finds Dillon at his cabin and tells him Kitty is in danger from Mannon, but his horse breaks a leg and Dillon has to go on alone. Kitty is held in a hotel room in Dodge by Mannon. She manages to throw his gun belt out the window just as Dillon arrives. The inebriated Mannon is then tossed out the window by Dillon. In the dusty night street he draws one last time, but Dillon is faster and shoots him dead. The townspeople gather at the scene as their former marshal leaves. Kitty wistfully watches from the window.
