The Horror!? 15: The Ape Man (1943)

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You may remember Bela Lugosi, mad scientist. At this point in his acting career the great man was reduced to playing a "gland expert" who should know better than to try self experiments, but tries his new "serum" (whatever it may be meant to achieve) out on himself anyway, only to mutate into a bad case of ape man make-up. His only hope to become fully human again is to kill people for their spinal fluid. So he and his assistant (a bad case of "man in shoddy gorilla suit") go on a killing spree.
It is equally sad and hilarious to watch Lugosi in his ape man get up, trying to imitate an apelike gait and playing the doomed scientist with as much pathos as he can.