With the aid of his volatile stallion Rex and oil rigger Happy Cardigan (Harry Carey Sr.), young Jimmie Williams (Frankie Darro) tracks the Vanishing Legion, a gang that framed his father (Edward Hearn) for murder. Led by a mysterious criminal mastermind known only as "the Voice," the outlaws plan to sabotage a local oil operation. Boris Karloff provides the voice of the unseen villain in this classic serial co-starring Edwina Booth.