The Vanishing Legion is a 1931 American Pre-Code Mascot film serial directed by Ford Beebe and B. Reeves Eason.

Stars Harry Carey and Edwina Booth were originally intended to be in the studio’s previous serial, King of the Wild, but had to be re-cast when the MGM film Trader Horn (1931) overran its schedule. Both also starred in the Mascot serial The Last of the Mohicans and Carey was the lead in The Devil Horse. An infection received while filming Trader Horn led to Booth’s retirement from acting.

Plot
Masked mystery villain The Voice is out to sabotage the Milesburg Oil Co. “Happy” Cardigan needs to successfully drill for oil before his contract with Milesburg expires or he goes broke. Jimmie Williams’ father, Jed (Edward Hearn), has been framed for murder. Secretary Caroline Hall appears to have an ulterior motive. The mysterious “Vanishing Legion” is also on the scene. Jimmie and Cardigan team up, along with Jimmie’s horse Rex, to defeat The Voice and solve the mysteries surrounding Milesburg.

Cast
Harry Carey as “Happy” Cardigan, drilling for oil before his contract expires
Edwina Booth as Caroline Hall, Hornbeck’s secretary
Rex the Wonder Horse as Rex, King of Wild Horses, whom only Jimmie can ride
William Desmond as Milesburg Sheriff
Frankie Darro as Jimmie Williams, whose father is wanted for murder, teams up with Cardigan to unmask The Voice
Philo McCullough as Stevens
Yakima Canutt as Yak
Joe Bonomo as Stuffy
Tom Dugan as Warren, a Milesburg Oil Company director
Lafe McKee as Hornbeck, a lawyer for the Milesburg Oil Company
Bob Kortman as Marno, henchman of The Voice
Paul Weigel as Larribee, a Milesburg Oil Company director
Olive Carey as Nurse Lewis
Boris Karloff as the voice of The Voice, the serial’s masked mystery villain