A posse discovers a trio of men they suspect of murder and cow theft and are split between handing them over to the law or lynching them on the spot.
When his cattlemen abandon him for the gold fields, rancher Wil Andersen is forced to take on a collection of young boys as his cowboys in order to get his herd to market in time to avoid financial disaster. The boys learn to do a man's job under Andersen's tutelage, however, neither he nor the boys know that a gang of cattle thieves is stalking them.
Honest Plush Brannon is a con-man thrown out of the Barbary Coast in San Francisco in the 1880s and headed for the gold rush region of Nevada. He discovers a real mine which lead to several complications.
Bolton has organized a feud between the Rork's and the O'Neil's. He has rustled cattle and killed a man putting the blame on Danny O'Neil. Tom Rork has found a bullet with markings on it that he hopes will clear Danny and bring in the real killer.
Opportunistic con man Dan Kehoe ingratiates himself with the cantankerous mother of four outlaws and their beautiful widows in order to find their hidden gold.
Bob Tyler has rustler trouble while driving a herd of cattle to the new owner, but he refuses to turn the herd over to Frank Kellogg. He has a run-in with Jean Polk, discovers she is the owner of the cattle, and is fired. With his friend, Barney McCool , Bob snoops around and discovers that Kellogg is behind the rustling.
When Matt Kilgore and his men frame and then hang an innocent man, Lige Saunders sends for his son Wild Bill Saunders who arrives to find his father shot by Matt's brother. When the brother is killed in his fight with Bill, Matt sends two fake Deputies to arrest Bill whom he then plans to hang. But Matt's sister, attracted to Bill, overhears the plan and rides for help.
The local school is causing Hoppy problems. First Bar 20 cattle are stolen when Hoppy investigates a problem there. Then the new teacher arrives and disrupts the routine of the Bar 20 hands. Later with the Bar 20 hands at graduation, the rustlers are poised to strike again. But there is dissension among them and this will lead to the break that Hoppy needs.
Escaped criminal "The Fox" hates Hoppy and a Rurales colonel for imprisoning him and lures Cassidy to Mexico in order to exact his vengeance.
Caldwell and Nixon have their men rob the stage and then critcize the Sheriff for not catching the robbers. With her father the Sheriff under pressure, Mary sends for Hoppy who finds the stolen money and sets a trap to bring in the entire gang.
Hoppy and Lucky are headed to South America to deliver a heard of cattle. Bay guy Ralph Merritt gets in their way. For a while.
Buck Colins heads a group of local ranchers who are trying to prevent the railroad from completing its line through their property. Till now they have been able to charge tolls on herds passing through. Hoppy goes undercover to expose them.
Hoppy goes undercover as a gambler from the East when Bar 20 cattle are stolen by unknown rustlers. Brennan/Talbot are twin brothers (one a casino owner, the other a rancher) and Hoppy believes they provide alibis for each other while one is out committing crimes. Hoppy gets a job in the casino to learn more but is exposed when a gambling gunslinger notices him.
U.S. Marshal Hopalong Cassidy is called when a town becomes overun with bad guys. Disguised as a member of a medicine show, Hoppy discovers that the ringleader is none other than sweet li'l ol' Ma Burton.
An evil gang is involved in both cattle rustling and the robbing of stagecoaches. Hoppy must stop them without help from the sheriff who turns out be a major outlaw himself.
A crusading newspaper editor recruits his old friend Hoppy to take the job of Marshall in a town rife with vice and murder directed at helpless miners.
On a cattle drive Hoppy, camp cook Windy, companion Lucky, and young Artie Peters encounter an eccentric professor. The professor professes to be searching for the evolutionary missing link, but in reality he is a cattle rustler who uses his dynamite to scatter the cattle in order capture some of them. Hoppy and Bar 20 guys ultimately capture the professor.
Hoppy's brother has been murdered and he is on the trail of the murderers. To get them he makes himself seem to be a wanted man.
Problems come in the form of one of Hopalong Cassidy's neighbors, but the matter is settled when Hoppy roots out the troublemaker.
The U.S. Army needs more horses for the Spanish-American War. Hoppy must turn his Bar 20 cowhands into Rough Riders to gather up the horses, and of course bad guys try to sabotage the operation.