A crazed scientist needs primates to conduct experiments to prove his own theory of evolution, so he organizes an expedition into the jungles of Borneo to capture the animals he needs.
When attacked by two dogs, Joe Gilmore leaves them on the desert to die. Later one of the dogs saves John Blake from drowning. Men arrive claiming the dog is killing their chickens. They want to kill the dog but John convinces them the dog's fate should be determined by a trial.
A young lawyer is elected mayor of the city and promises to rid it of the corruption it's famous for. The problem is that most of the corruption he's vowed to eliminate is caused by the crooked political machine that helped elect him.
When Mark 'High Gear' Sherrod (Murray) looses his nerve, the race car driver takes a job driving a taxi, but when he befriends a cute reporter and the young handicapped son of a deceased driver, he attempts to return to the track.
Kincade and Blake cause a mail plane carrying a payroll to make a forced landing in the desert. When they try to get the money, prospectors Ted and Si drive them away. With the pilot shot, Ted takes over as pilot figuring another attempt will be made and this time the Sheriff will be there.
Cowboy is hired by an archaeologist to help find "Hidden Valley", where an Indian gold treasure is supposed to be buried. Just when he finds it, the archaeologist is killed, and the cowboy his charged with his murder.
When legendary hunter Bob Rainsford is shipwrecked on the perilous reefs surrounding a mysterious island, he finds himself the guest of the reclusive and eccentric Count Zaroff. While he is very gracious at first, Zaroff eventually forces Rainsford and two other shipwreck survivors, brother and sister Eve and Martin Towbridge, to participate in a sadistic game of cat and mouse in which they are the prey and he is the hunter.
Steve Drexel voluntarily strands himself on a deserted island on a bet. He intends to re-create civilization and carves a miniature city of 52nd Street and Park Avenue out of the jungle. Drexel is befriended by his dog, a native monkey, and a wild goat that is captured in one of his traps. He attempts to cultivate a native as his Man Friday from Robinson Crusoe, but fails as the native escape.
When a young South Seas sailor falls overboard, the beautiful daughter of a Polynesian king dives in and saves his life. Thus begins the romance of Johnny and Luana. Though Luana is promised to another man, Johnny whisks her away, and for a brief time the lovers live very happily together. But, when a local volcano threatens their lives, Luana knows that she must sacrifice herself to the volcanic gods in order to save her island.
The Wrecker wrecks trains on the L & R Railroad. One of his victims is Larry Baker's father. Baker wants to find the evildoer, among a host of suspects, but it will be difficult since the Wrecker can disguise himself to look like almost anyone
Canadian Mountie Mason is sent south of the border to look for a horse thief with only a watch chain for evidence. He befriends young Andy and when Calhoun hits Andy, Mason and Calhoun fight. In the scuffle Calhoun's watch with the missing chain is dislodged. Mason then sets out to bring in Calhoun and his gang.
Jim Gray is looking for the gang leader known as the General. When Neil Denham is murdered, Jim assumes his identity. He and his pal Squint Saunders then try to join the gang. But they get captured and Jim is told he can join up only if he kills his friend.
Heading west, Ken and Bouncer end up at the Brooks ranch where Ken is to ride Tarzan in the big race. But both the Sheriff and Edmonds are after him and he must hide both himself and the horse until race time.
Escaping from the Sheriff, Jim and Cookie decide to go straight. But when they meet their old cohort, The Blanco Kid, he tells their new boss they are outlaws and they are in trouble again.
Head railroad man Dan is as ugly as he is honorable. When he spots a drifter who'd hopped a freight held up by a landslide, Dan offers the man a job; then he finds the man was a railroader, too, and takes him under his wing. Engaged to Mary, Dan doesn't notice the growing attraction between his protégé and his intended but focuses instead on running the railroad.