A scientist trying to cure a girl from a crippling illness discovers that human spinal fluid is the key ingredient to the cure. When he kills an escaped gorilla in his home, he uses its hide as a costume to procure the spinal fluid.
I don't know if this was the first horror movie to use spinal fluid as its secret ingredient, but I haven't seen an earlier one. This would be Karloff's only horror movie with Monogram, and it's interesting to compare this with Lugosi's Monogram horrors. One thing I notice is that the movie seems more geared to Karloff; the doctor is given a kindly quality that would most likely be absent if Lugosi was playing the role. I really responded to this one as a kid; I recall almost crying at the end of it. Nowadays, I see that it's pretty silly, but I enjoy it all the same.