"The Strange Case of Doctor Rx" is billed as a horror film, but it really isn't. It's an all too obvious murder mystery with just one horrific scene that seems spliced in from another movie altogether. William Nigh probably relished the chance to direct at a major studio instead of the salt mine of Monogram, but though the film has handsome production values Nigh brings to it all the trademarks of his Monogram work: dullness, no sense of pace and an unwillingness to tell any of the actors they're overacting. Though billed second, Lionel Atwill is barely in the movie at all until the final reel. The title is obviously intended to evoke memories of Atwill's great film "Doctor X" from Warners in 1932, but the comparison only makes this film seem even worse than it is. The great Black comedian Mantan Moreland easily steals the movie out from under all the white stars; he's genuinely witty and a charismatic screen presence, and he gets the first line of dialogue as well as the final gag. Next to Moreland, the most entertaining aspect of the movie is Patric Knowles' gorgeous Art Deco apartment, where all too much of the film takes place. Why Universal and TCM chose this for their "Cult Horror Classics" boxed set instead of the neglected 1934 masterpiece "The Man Who Reclaimed His Head" (also with Atwill) or the interesting 1942 Edgar Allan Poe adaptation "The Mystery of Marie Roget" (also with Knowles) is a bigger mystery than the supposedly "mysterious" identity of Dr. Rx in this plot.