Behind the Mask is a typically virile Jack Holt vehicle, with the hero at one point shooting himself in the arm to establish an alibi! Holt plays a federal agent named Hart who has himself planted in jail as a convict to get the goods on a drug syndicate. Befriending small-time gangster Henderson (Boris Karloff), Hart follows the trail of clues to unmask the head of the syndicate, who turns out to be the supposedly respectable Dr. Steiner (Edward Van Sloan). In the rip-roaring climax, Steiner prepares to perform an "operation" on Hart, gleefully informing his victim that his chances for recovery are next to nil. Because of the presence of Boris Karloff and Edward Van Sloan in the cast, Behind the Mask was included in Screen Gems' "Shock Theater" TV package, even though there's nothing really horrific in the film.