Tom and his teenaged friend exchange shots with a posse. Tom grabs Billy and rolls down the hill. The marshal and his posse move in and find blood, but no sign of Billy. Tom rides into Sweetwater where Philo sees him unload Billy from his horse and take him into the Red Ox. He tells Jack and Shifty to tend to the teenager's wounds.
The next morning, Bret and Cy ride into town to get material for the new barn that they're building from a Bradlee kit. Bret drops Cy off and the foreman sees Marshal Phillip Mondragon and his posse ride into town. Tom also sees them and tells Shifty that they'll have to wait until later to take Billy to the doctor. Meanwhile, a man claiming to be Terrible Fred McShane storms into the saloon and demands a drink. Philo watches him and says to one of the bar customers that they need to watch the man.
Mondragon goes to see Dowd and explains that he and his men ran into some scavengers up the road. The marshal complains about his injuries and asks the name of the doctor in town. He then asks if he could stay over for a few days to rest.
While he waits for his supplies, Bret talks to Rodney and admits that he's nervous that the townspeople will be coming to help him raise a barn. Rodney is more interested in getting card-dealing tips. Bret sends him on his way and collects his supplies, but one of the posse members, Beech, recognizes him. Things are tense for a few moments but then Beech lets Bret move on.
At the ranch, Tom comes to see Bret and ask for his help. He wants to put his friend up at the Lazy Ace. As they go inside, Bret discovers that Tom's friend is already there. Bret ushers Tom outside and complains that his partner has brought in William Bonney: Billy the Kid. When Bret hears that Mondragon is in town, he tells Tom to get Billy out before the people arrive for the barn-burning. Billy limps out and says that Bret is right. Tom reluctantly ushers Billy to the wagon and ignores Bret's sudden advice. The gambler finally gives in and agrees to let Billy stay overnight until Doc Vetter from town can arrive and remove the bullet.
Mondragon learns from his men that Bret is in Sweetwater. Mary Lou arrives and points out that Mondragon's claim about going to Ft. McHenry is false. She figures that Mondragon is after Billy the Kid and leaves. Mondragon tells his man Beech to follow her. When Dowd demands some answers, Mondragon tells him that Billy killed his last friend during his escape and he won't tolerate any interference. He goes to the Red Ox and offers to buy Tom a drink. Unable to leave gracefully, Tom agrees.
Philo invites McShane to his cellar room and they share drinks. The scout has McShane talk about his exploits and the $1,500 reward on his head. McShane informs him that he's after Bret Maverick with the plan to kill someone famous. He draws a gun on Philo and demands that he helps. Philo agrees and then knocks him out when his back is turned.
Mondragon talks about his family and his noble blood. Tom finishes his drink and starts to go, but Mondragon points out that he has a scratch on his cheek from a bullet. Before the marshal can pursue the matter, Philo arrives and offers to take Mondragon to the man he wants. Mondragon agrees to loan one of his men to accompany Philo. Tom rides off and one of Mondragon's men watches him go.
Philo takes Mondragon's man to his cellar only to discover that McShane has left.
Two of Mondragon's men, Lester, and Red, follow Vetter to the Lazy Ace. Red wants to go back to notify Mondragon, but Lester wants to collect the reward himself. They burst in and discover that Vetter is tending to Cy's leg injury. They search the ranch but find nothing. Once they leave, Bret takes Vetter to the cellar where Billy is resting. Vetter removes the bullet successfully. Afterward, Billy talks about how he got started as an outlaw when he went up against crooked lawmen who gunned down his friend during the cattle range walls. Now Billy plans to get revenge. Tom leaves and Billy explains that he only met Tom when they ran across each other in the hills the day before.
Afterward, Bret talks to Tom and suggests they tell Billy there's no percentage in revenge. Tom figures that Billy wouldn't listen. McShane fires on Bret and the two men take cover. Bret sneaks up on McShane and captures him, while Tom captures Philo. They tie McShane and Philo up, and the Indian scout insists that he came out there to warn Bret. He explains incorrectly that Mondragon is after McShane. The supposed outlaw admits that he might have made up some of his tales. McShane finally tells them that he worked as a butcher until he got sick of the job, sold his business, and moved west. They're interrupted when Mary Lou arrives at the ranch, alone.
Mondragon chews out Red and Lester, but Beech has spotted Mary Lou heading for the ranch. The posse rides out.
Mary Lou has figured out that Billy is at the Lazy Ace. Bret tries to talk her out of pursuing the story by offering an interview, but Mary Loup persists. They take her in to see... McShane. He tries to bluff it out but she quickly realizes that McShane is lying. Mary Lou threatens to go back to town and publish her story, but Tom says she won't. He takes Mary Lou in to see the sleeping boy and asks if Billy's life is worth her story.
The next morning, Mary Lou helps Tom and Billy leave in the wagon. Later, Mondragon and his men watch from a nearby hill as the townspeople arrive for the barn-raising. As they work, Bret spots the lawmen but figure it's safest to let them be. However, Tom is forced to return after discovering that Mondragon has blocked all the routes out of Sweetwater. Billy goes into the ranch house but collapses just inside the door. Philo comes in and sees that something is up, and tries to leave. Tom draws a gun on him and tells him to stay.
As the townspeople puzzle out the plans to the barn, Mondragon and Lester ride up. Bret tries to stall him but Mondragon insists on going inside to search the house. The marshal pushes past him and goes inside. Mary Lou and McShane are preparing lunch, and she secretly gestures to Bret that Philo is tied up behind the couch. Bret keeps him hidden and Mondragon finally goes to see the barn. Philo manages to wriggle free of his bonds and runs to warn Mondragon, but Tom knocks him out and gets him back inside.
While Tom takes Philo to the shed where Billy is hiding, Mary Lou is relieved when Mondragon rides off. However, Bret thinks that it was too easy. Meanwhile, Tom tells Billy that he knows all about revenge and that he gave it up a long time ago.
As the townspeople work on the plans, Bret spots Mondragon and his men on a nearby hill. He tells Tom that they'll have to play a version of the shell game and get Mondragon to go in the wrong direction. Bret offers to take out Billy but Tom refuses until his partner insists.
As Mondragon watches, Bill and Tom load up two wagons. The townspeople come over and try to warn Bret that there's been a mix-up in the plans for the barn, but he tells them to deal with it and rides off. Mondragon and his posse follow Bret and pull him over, but Bret reveals that he has McShane hidden in the back of the wagon.
Dowd and his men intercept Bill and the sheriff insists that it's his jurisdiction. He opens a crate and discovers that Philo is tied up.
In the meantime, Mary Lou has smuggled Billy out in her wagon. She suggests that he ride off to Mexico and give up his quest for revenge. Billy says that he'll consider it and rides off.
Bret returns to the ranch and discovers that Berklee sent the wrong plans. He now has a church on his ranch.