Dr. Bernard Adrian is a kindly scientist who seeks to cure a young woman's polio. He needs human spinal fluid to complete the formula for his experimental serum. Meanwhile, a vicious circus ape has broken out of its cage, and is terrorizing the townspeople. Can there be a connection?
| Actor | Role |
|---|---|
 Boris KarloffDr. Bernard Adrian | Dr. Bernard Adrian |
 Maris WrixonFrances Clifford | Frances Clifford |
 Gene O'DonnellDanny Foster | Danny Foster |
 Dorothy VaughanMother Clifford | Mother Clifford |
 Henry HallSheriff Jeff Halliday | Sheriff Jeff Halliday |
 Selmer JacksonDr. McNulty | Dr. McNulty |
 Gertrude HoffmannJane the Housekeeper | Jane the Housekeeper |
 Jessie ArnoldMrs. Brill | Mrs. Brill |
 Billy BletcherShort Mustached Posse Man | Short Mustached Posse Man |
 Harry C. BradleyQuinn | Quinn |
 George ClevelandMr. Howley | Mr. Howley |
 Ray CorriganNabu | Nabu |
| PPauline DrakeYoung Girl | Young Girl |
| MMary FieldMrs. Mason | Mrs. Mason |
 Gibson GowlandPosse Member | Posse Member |
| JJulia GriffithTownswoman | Townswoman |
 I. Stanford JolleyApe Trainer | Ape Trainer |
| SStan JolleyBoy in Soda Shop | Boy in Soda Shop |
| JJack KennedyDep. Archie Tomlin | Dep. Archie Tomlin |
 Donald KerrCitizen Carrying Mauled Ape Trainer | Citizen Carrying Mauled Ape Trainer |
 Philo McCulloughHenry Mason | Henry Mason |
| BBuddy SwanWillie Brill | Willie Brill |
 Minerva UrecalTownswoman | Townswoman |