Dr. William Jekyll (Denis de Marne) is a brilliant scientist whose revolutionary theories on the nature of human evil have earned him derision from the more conservative corners of his profession. The devoted doctor has developed a serum that isolates and identifies the part of the brain that produces violent behavior, and is working on another chemical that will block all antisocial desires, effectively curing evil once and for all. Unfortunately, Jekyll gives the formula for this untested antidote to his assistant Smithers (Berwick Kaller), who clumsily ruins the paper and quietly recopies the instructions from memory. Believing that he possesses both working drugs, Jekyll uses himself as a live human guinea pig and quaffs the serum that will identify the evil lurking in his brain. Of course, something was left out when Smithers rewrote the formula for the antidote, which means that the crusading doctor is transformed into Mr. Blood, a sadistic, lustful creature with grey-green skin and ferocious eyebrows. He charges into the night to quench his thirst for cruelty, and meets April (Julia Stratton), a nightclub floozy who accepts the money Mr. Blood offers for the right to humiliate and abuse her. Murder, dismemberment, and wild S & M orgies also figure in this vicious libertine's search for satiation. Jekyll remembers nothing after recovering from his fits of violence, but his sister, fiancée, and the students he teaches all notice that his behavior is becoming very strange. The doctor's evil alter ego begins surfacing uncontrollably and at random, threatening the safety and sanity of his loved ones, as well as Jekyll himself.