Boris Karloff portrays Dr. Adrian, who lives in a small town and does unorthodox experiments in his home. He was kicked out of the University 25 years before and ever since then has tried to come up with a serum which would undo paralysis. One of his neighbors is a young paralyzed young woman, who he views as a daughter. He promises her that he will cure her polio and she will walk again. However, he needs to get his serum from human spinal fluid.A circus has recently come to town and its abused gorilla escapes. It ends up mortally wounding its keeper-and eventually ends up breaking into Dr. Adrian's house. Adrian manages to kill it with a knife-or so it seems.The ape, though, is later seen killing people about the town, such as the mean old business man who nobody will miss. When the police gun down the gorilla at the end of the movie they find out that it is really Dr. Adrian in an ape costume-he was killing people in order to get their spinal fluid so he could cure the girl and thus prove his theories.