For many years a curtain of mystery has surrounded the activities of "Mad" Martin (Walter Soderling), an eccentric old recluse living in Whisper Canyon. His only companion is Brillant, a dwarfed horse with a highly developed mentality. And his only contacts with the outside world are his pretty niece, Ann Martin (Helen Deverall), his lawyer Frank Nolan (Charles Miller) and Ann's sweetheart Freddie (George J. Lewis), a stagecoach driver who occasionally delivers mysterious packages of book to Martin's cabin. He is found murdered in the hills near his cabin and Brillant is missing. The Three Mesquiteers, Tucson Smith (Bob Steele), Stony Brooke (Tom Tyler) and Lullaby Joslin (Jimmie Dodd) are riding through the territory and capture Brillany, but are quickly pounced upon by a posse led by Sheriff Pillsbury (Hal Prive), who accuses them of murder and has them jailed awaiting trial. That night, three men kill Deputy Bets McGee (Budd Buster), steal Brillant from the sheriff's barn and haul the little horse away in a stagecoach. Realizing that the strangers are connected to Martin's murder, the Mesquiteers break jail and give chase. The stagecoach is overtaken and although two of the henchmen escape, the third is captured. The Mesquiteers tie him up and wait for Sheriff Pillbury's posse to catch up. But when the posse arrives it is discovered that the prisoner has been stabbed to death by his pals to keep him from confessing. One again the Mesquiteers are accused of murder and, once again, they make an escape and take Brillant with them. Many more plot twists and complications follow before the mystery about Brillant is solved.