A mad scientist tests his growth serum on an ape.

After a year in the Ugandan jungle, botany professor Charles Decker returns to England with a revolutionary growth serum and a small chimpanzee named Konga. Decker and his assistant, Margaret, begin injecting Konga with the serum, and he grows. When the university dean reproves Decker for discussing his discovery with the press, Decker, now mad with power, hypnotizes Konga and commands him to kill the dean. In a similar fashion, Decker also disposes of a rival botanist, Professor Tagore, and Bob Kenton, the boyfriend of a pretty student, Sandra, with whom Decker has fallen in love. Eventually Margaret learns of Decker's interest in the young girl, and in a fit of jealousy she gives Konga an overdose of the serum. Konga grows to an overwhelming size, starts a fire, hurls Margaret into the flames, causes Sandra to stumble into the tentacles of a carnivorous plant, and scoops up Decker in his massive hand. Confused, baffled, and still clutching Decker, Konga staggers through the London streets, terrifying the populace. Armed troops corner the beast and bombard him with rockets and machine-gun fire. In pain and bewilderment, Konga hurls Decker at the crowd, crashes to the pavement, and shrinks to his original size as he dies.