Dr. Renault's secret is a serum which, along with a little cutting into the brain, turns a jungle ape into a semi-human. The ape, now regarding himself through a human's eyes, resents what has been done to him, and his rage turns murdererous. The first person he vents that rage on is, of course, Zucco who gave him the sensitivity to pity himself. The director, Lachman, originally was a painter, with works hanging in the Chicago Art Institute as well as in the Prado in Spain. He entered films as a set designer in 1922 and spent the next 20 years as a moviemaker before returning to painting. Art director Juran, who dressed this film up to give it some class, went on as a director of interesting poverty-row science fiction films, including ATTACK OF THE 50-FOOT WOMAN, THE DEADLY MANTIS and 20 MILLION MILES TO EARTH.