Fists fly as the mysterious Vanishing Legion, led by a shadowy villain known only as The Voice (Boris Karloff), sabotages an oil-drilling site. Digging for the truth is rigger Happy Cardigan (Harry Carey), whose investigation of the Legion's activities is complicated by his efforts to help young Jimmie Williams (Frankie Darro). Jimmie's father, Jed, was framed for murder by the same marauding gang that Cardigan is after. At the center of this tangled web is beautiful oil well owner, Caroline Hall (Edwina Booth). Cardigan is attracted to Caroline but suspects she may be working for the very group launching raids on her property. When the Voice orders the young woman's death, Cardigan risks his own neck in his struggle to uncover the real murderer and save Caroline from the Vanishing Legion.
In the grand tradition of Hollywood serials, The Vanishing Legion overflows with cliffhanging chapter endings involving out-of-control trucks, stampeding horses, falls from the tops of oil rigs, and wild and wooly fistfights. Director B. Reaves Eason helmed such serials as The Last of the Mohicans (co-directed with Ford Beebe), The Phantom Empire (with Gene Autry), Undersea Kingdom and The Phantom (with Tom Tyler). The screenwriter of The Vanishing Legion, Ford Beebe, went on to become a first-rate serial director in his own right. Some of his most popular entries include all three Flash Gordon serials with Buster Crabbe, The Phantom Creeps (with Bela Lugosi), Buck Rogers, The Green Hornet and Universal B features including Night Monster and The Invisible Man's Revenge.