In the first of two proposed serials for Mascot Pictures, Western hero Ken Maynard goes up against a murderous fiend known as "the Rattler." Wearing a strange disguise consisting of eye glasses, a fake nose, and crepe-hair mustache, the Rattler, aka "the Menace of the Mountain," attempts to control the mountain -- and its hidden gold -- from a secret cave filled with strange electronic gadgets. Maynard is Ken Williams, a young cowboy coming to the aid of Jane Corwin (Verna Hillie), whose railroad worker father (Lafe McKee) was the Rattler's first victim. Just as in a previous Mascot serial, The Hurricane Express (1932), the masked villain of Mystery Mountain uses a seemingly endless supply of rubber masks that enables him to perform his skullduggery disguised as almost every member of the cast. He is finally brought to ground in chapter 12, "The Judgment of Tarzan" ("Tarzan" being Maynard's faithful steed), and is revealed to be supposedly solid citizen Edward Earle. The denouement, of course, was a typical Mascot "cheat," the masked villain having up to that point been played by Edmund Cobb. Maynard, whom Mascot producer Nat Levine had gotten on the cheap at 10,000 dollars a week, proved almost not worth the trouble he created. The difficult star demanded that the serial be filmed at his old stomping grounds, Universal City, and kept changing the script and direction to suit himself. Although Mystery Mountain proved the most successful Mascot serial up to that time, Levine had had enough of the obstinate Maynard and replaced him with newcomer Gene Autry in The Phantom Empire (1935).