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Under Cover was a high-grade detective yarn with heavily comic undertones. Neither heroine Ethel Cartwright (Hazel Dawn) nor hero Steven Danby (Owen Moore) were quite what they seemed to be, and much the same could be said for the rest of the characters. The studio's publicity package was careful not to divulge any vital plot details, for fear of giving away the surprise ending. No one who had seen the Roi Cooper Megrue play upon which the film was based was surprised by its outcome, but even these in-the-know moviegoers stuck with the picture from first reel to last. The reviewer for the trade magazine Variety honored the producers' wish to keep the outcome of Under Cover under cover, though he was moved to comment that this was one of the few films in which the "wealthy" characters behaved as if they really were wealthy.