British botanist Dr. Charles Decker comes back from Africa after a year, presumed dead. During that year, he came across a way of growing plants and animals to an enormous size. He brings back a baby chimpanzee, named Konga, to test out his theory. Decker goes insane after he discovers a serum that turns his chimpanzee subject into a ferocious gorilla-sized ape. To further his hideous experiments, he mesmerizes the ape and sends it to London to kill all of his enemies who have more credit in the scientific community than he already has. Among his targets are Dean Foster, Professor Tagore, and Bob Kenton, the lover of Sandra Banks, the woman that the doctor wants for himself.
During a field trip to the woods with a group of his students he makes an inappropriate advance to Sandra. He is later confronted by her boyfriend Bob and, although seeming to concede to Bob, sends Konga to strangle him.
Decker's assistant and lover Margaret spots his affection for Sandra and attempts to get even by giving Konga an enormous amount of the strange serum and turns him into an enormous monster, although she becomes his first victim.
Just before going on a rampage, the super-sized ape grabs Decker in one of his enormous hands, while Sandra is eaten by Decker's carnivorous plants. His rampage comes to a stop when he (and Decker) are killed by the British army.