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In this adaptation of the 1886 story "Strange Case of Dr. Jekyll and Mr. Hyde," psychologist Charles Marlowe (Christopher Lee) invents a drug that helps release the inhibitions of his patients. But when he tests it on himself, the concoction reveals a different side of Marlowe - an evil side named Edward Blake who spirals into a life of crime and, eventually, murder. His friend Frederick Utterson (Peter Cushing) suspects when grisly killings take place.