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, Konga, to test out his theory. Decker goes insane after he discovers a serum that turns his subject into a ferocious -sized ape. To further his hideous experiments, he mesmerizes the chimp and sends it to to kill all his enemies who want more credit in the scientific community than he already has. Among his targets is Dean Foster, Professor Tagore, and Bob Kenton, the lover of Sandra Banks, the woman the doctor wants for himself.
After Konga strangles Bob Kenton to death, Decker attempts to make Sandra his own. This doesn't sit well with Margaret, the botanist's assistant and current girlfriend, who 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's arm is eaten by Decker's carnivorous plants. His rampage comes to a stop when he and Decker are killed by the British army. Upon his death, Konga reverts to a chimpanzee.