Originally written as a stage vehicle for corpulent character actor Macklyn Arbuckle, Ernest Day's The Roundup was first filmed in 1920 with Fatty Arbuckle (no relation) in the lead. By the time the film was remade in 1941, Arbuckle's character, a roly-poly frontier sheriff named Slim (!), was refashioned as a supporting role, with Jack Benny's radio announcer Don Wilson essaying the part. The plot, however, remained fairly intact: Upon hearing that her fiance Greg (Preston Foster) has been killed, Janet (Patricia Morison) agrees to marry rancher Steve (Richard Dix) on the rebound. On the day of the wedding, who should show up but Greg, determined to raise as much Hell as humanly possible
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.
Joe Weller has instigated a conflict over water rights between two ranchers. The idea is to have the ranchers do each other in then move in and take over. Hoppy and the good guys won't let this happen.
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.
Hoppy and new sidekick California Carlson head to California to help out Lucky Jenkins.
Hopalong Cassidy, boss of the Bar 20 ranch in Texas, rides down the Camino Real in the New Mexico cattle country near Alamogordo, in response to an urgent message from his lifelong sweetheart, Nora Blake, who is in serious trouble. Before he and his saddlemates, "Lucky" Jenkins and "Pappy", can reach her ranch, they are stopped by Clay Allison, a cattle-rustler who is in almost complete control of the district, and wants to extend his holdings by seizing Nora's cattle and driving her out. Seeing Cassidy as a menace to his plans, he has him arrested on a trumped-up charge. Cassidy and his pals shoot their way out of the trouble and reach Nora;s ranch where they learn that Allison's henchmen have murdered her foreman, Tom Dillon, and Allison has sent for a crew of outlaws on the Texas border.
Ben Wade and his partner Frosty return to Bellounds' ranch where twenty years earlier Wade was wanted for murder. Unrecognized, he gets a job on the ranch and soon becomes involved in Folsom's cattle rustling and a chance to settle an old score.
Bat Masterson, who after failing to secure a job as a newspaper reporter becomes marshal of Dodge City. Preferring socializing to peacekeeping, Masterson falls in love with Dora Hand, the obligatory golden-hearted chorus girl whose concern for the welfare of her fellow citizens at time reaches Madonna-like dimensions. When Dora is shot down cattle baron King Kennedy, Masterson begins taking his job seriously. After taking care of Kennedy, Masterson determines to enshrine the memory of Dora, whose efforts to clean up Dodge City were largely ignored by the "decent" townsfolk.
A railroad man and the owner of a freight line battle for control of a crucial mountain pass.
Cattle rustler Nevada dreams of living like an emperor in the West. Hoppy and the Bar 20 boys aim to put an end to his dream.
An evil ranch foreman tries to provoke a range war by playing two cattlemen against each other while helping a gang to rustle the cattle. Each cattleman blames the other for missing cattle. With the help of Bill Cassidy (Hop-along, because of an earlier bullet wound) and Johnny Nelson, the warring cattlemen join forces to do in the outlaws.
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.
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.
Julie's husband has been murdered and land agents want her to sign away her property rights. Hoppy warns against this but she does so anyway. It looks as though she will be unable to deliver the timber called for in her agreement. Hoppy has to make the lumber deal happened and solve the murder.
A town bedeviled with outlaws sends for Hoppy, Lucky and California after their own vigilante committee fails to solve the towns problems. Hoppy discovers that the bad guys are led by the town boss, and so are the vigilantes.
Posing as a cattle buyer, Hoppy crosses over into Oklahoma where the Jordan brother's and their outlaw gang operate outside the law. After receiving an unfriendly reception when he finds them, he, California, and Johnny rustle their cattle and drive across the river into Texas. He hopes they will cross over to retrieve their cattle and then he can arrest them.
European bad guy Baron Bendor leads some local townsmen in a plot to obtain horses through theft. Hoppy and his sidekicks Lucky and Speedy must find and expose the horse thieves.
Hoppy and Lucky have been called in to investigate a series of stage holdups. The robbers are taking gold from Colby's mine and Hoppy suspects it may be ex-outlaw Colby himself. When Speedy strikes gold, Hoppy borrows it and announces a gold shipment hoping to catch the gang and their leader.
When three Texas Rangers try to investigate kidnapped Mexicans being used as forced labor in the mines of Silver Bullet, they are framed for murder by the town's corrupt sheriff.
As rustled cattle have mysteriously disappeared, Johnny sends for his friend Hoppy, Hoppy arrives and immediately suspects Dan Slack. Realizing his telegram about Slack was intercepted, he locks up the operator Lafe knowing he can escape. Tailing Lafe he finds a secret entrance to a mine and inside finds the missing cattle. But Slack's men also find him just as the cattle are stampeded through the mine shaft.