The Phantom Empire (1935)

This movie would be a classic of its type, if there were anything else in its type. The ostensibly peaceful underground kingdom of Murania actually exists on the slave labor of robots, who are planning a revolt. Periodically, the Muranians dress as "Thunder Riders" and after rocketing to the surface, come out of a cave to terrorize the Surface People (us). But they can't terrorize a gang of kids known as the "Junior Thunder Riders", whose motto is "To the Rescue!" They shout this while wearing buckets on their heads, in imitation of the gas masks of the Muranians. Mendacious scientists have detected radium beneath the surface of Gene Autry's ranch, but they don't know that the radium is coming from an underground kingdom. The scientists keep kidnapping Gene so that he won't make it to his weekly radio show and hence won't get the paycheck that he depends on to make his mortgage payment. The mortgage payment is the engine that actually keeps all these balls in the air, and as in all good westerns, the bankers are the real villains. Gene is so cool as he handles the Thunder Riders, evil scientists, mendacious bankers, cruel but oddly flirty Queen Tika, and surprisingly clumsy robots, while always having time for a kind word to the Junior Thunder Riders and Smiley Burnett. You can tell that he lives by the Cowboy Code.