Lost in the African jungles for over a year, Dr. Charles Decker (Michael Gough) has returned to England with Konga, a baby chimpanzee. Disappearing into his lab, the mad botanist begins work on what he believes will be his greatest achievement. Having witnessed a Baganda witch doctor's use of a rare carnivorous plant to produce accelerated animal growth, Decker injects Konga with the same serum. Using the gorilla-sized chimp to brutally murder his enemies, Decker himself soon becomes victim to Konga's uncontrollable rage. Grabbing the scientist, the berserk chimp - who's grown to a monstrous size - breaks out of the lab and rampages through London, a city whose salvation rests with the military, who have been ordered to destroy Konga at all cost.
After a trip to Africa, mad-scientist Dr. Decker returns with ability to make plants and animals grow to an amazingly hefty size. When he tests his new developments on a baby chimpanzee, he unwillingly creates a gargantuan savage gorilla that wastes no time in terrorizing the streets of London!