In New Mexico territory, on his way to an apprenticeship in Lincoln, Tom Brewster, a correspondence school law student from Oklahoma, rides into the corrupt town of Blue Rock to mail his examination papers. The post office is closed because it is Election Day, but alcohol flows freely in the saloon owned by incumbent mayor Barney Turlock, who buys his victory. After Tom convinces postmaster Wally Higgins to post his letter, the elderly justice of the peace, Pop Pruty, also posts a letter addressed to the territorial marshal, which gives Wally a start. During the festivities, Tom ties in a horse race with tomboy Katie Brannigan, then loses to her in a shooting match, as he never carries arms. Afterward, Turlock offers him a high paying job as sheriff, a position vacant since the recent death of the previous lawman, but Tom declines the offer. Back outside, Tom and Katie compare the different philosophies of their respective fathers. While Tom's father taught him that handling a gun leads to getting shot, Katie's father believed that a fast draw is the best life insurance. Although many in the town are against Turlock, Katie vehemently defends him, because she believes he treated her father well. After Wally tells Turlock about the letter mailed by Pop, which he claims was in the handwriting of the deceased sheriff, news reaches town that the mail coach was robbed. Tom must retake the test as his exam papers have been stolen, so to support himself in the meantime, he accepts the sheriff's position. While he settles in at the sheriff's office, Katie comes in, offended that he would try to fill the boots of her father, who was the previous sheriff. She tells him that, Brannigan was ambushed and killed while trying to catch cattle rustlers. Although Pop seems to know something more about Brannigan's demise, he is careful not to reveal anything. In the days that follow, Tom's amiable chatter is his best tool for keeping the peace, as he sweet-talks gun-wielding drunks into good humor. While Turlock, who also owns a ranch, secretly instructs his foreman, Pete Martin, to steal neighboring cattle, Tom teaches rope tricks to some local boys. One of the boys, the macabre-loving Johnny Neil, says he found Brannigan's body in a dry gulch outside town. He shows Tom the location and tells him Brannigan had been shot through the heart. Tom finds it interesting that crack-shot Brannigan died with his gun in his holster. After checking the site, Tom finds a horse's hoof print and later matches it to Pete's horse, then concludes from other evidence that Brannigan was moved to the spot after his death. Later at the saloon, Tom mentions his findings to Turlock and waits to see his next move. Soon after, Turlock's cousin, the notorious Billy the Kid, rides into town. Both Pop and Wally warn Tom to leave, but Tom goes to the saloon when summoned by Turlock. After their introduction, the Kid works hard to pick a fight with Tom by calling him a coward and shooting around him. Tom chats amicably and counts bullets until he thinks the Kid has shot all six, then demands to be handed the gun. In a temper, the Kid shoots his sixth bullet and departs in frustration, leaving Tom surprised to be alive. Tom then tries to arrest Turlock for serving alcohol to the under-aged, eighteen-year-old Kid. After asking to speak in private, Turlock explains to Tom that Brannigan had gambling debts and was paid to ignore cattle rustling in the area. As proof, he produces Brannigan's signed confession and threatens to make it public, thereby hurting Katie, unless Tom backs off. Having passed his second exam, Tom resigns and packs for Lincoln, but Pop guesses the real reason for his planned departure. He tells Tom that the confession, which he assumes Turlock showed him, was the item he mailed the first day Tom was in town. Pop further reveals that Brannigan was trying to break free of Turlock and asked Pop to mail the confession if he was killed. Asked why he never spoke up sooner, Pop says that he has lived to be seventy-two by keeping quiet. Tom leaves, but is intercepted outside of town by Katie, who has learned the truth about Brannigan, and convinces him to return. Katie forges another envelope, similar to the first, which Pop mails. After learning about it, Turlock, fearing it is another confession naming him as the rustlers' ringleader, sends his henchmen to steal it from the mail coach. However, they are captured by a waiting posse led by Tom. To avoid being hanged, Pete confesses that Turlock killed Brannigan and ordered him to move the body to the gulch. Meanwhile, Turlock tries to kill Pop, but Katie intervenes. Knowing Katie will not shoot him in the back while he appears to be unarmed, Turlock escapes, but Johnny slows his departure with his lasso. Tom and Katie together capture Turlock outside of town. Later, after promising Katie that he will set up his law office in Blue Rock, Tom leaves for his apprenticeship.
