In the late 1800s London, surgeon Dr. William Jekyll (Dennis De Marne) procures the corpse of a murderer and rapist, hoping to prove his theory that he can isolate and treat the part of the brain associated with evil behavior. Although he is confident in his research, the medical establishment disputes his radical ideas, thus isolating Jekyll to a home laboratory. Regardless, Jekyll is excited by the promise of curing murderers and sex criminals and eagerly demonstrates to his six students, Victoria, Lucas, Webb, MacGregor, Jones and Murphy in a local classroom a formula that, when injected into the corpses brain, locates the evil sections by turning them a green hue. Although one student Murphy (Jeremy Brooks) walks out, stating that tampering with the soul is the dominion only of God, his other five students, as well as his assistant Smithers (Berwick Kaler) and loyal Victoria (Jennifer Summerfield), are committed to his experiments.That night, Jekyll, having already worked with live laboratory animals, finalizes his antidote to change the behavior then shows the written formula to Smithers and asks him to copy it. While Jekyll is out of the room, Smithers accidentally spills liquid on the document, which he then attempts to rewrite. Although he realizes that his notes might not be accurate, Smithers is too embarrassed to tell Jekyll the truth and leaves.Late that night, Jekyll's ever patient and loving fiancee Mary Ann Marsden (Gay Feld) pries him from his laboratory to attend their engagement party at her family's house. Over dinner, Mary Ann's brother, Dr. Oliver Marsden explains to their father, who also is a doctor, that both his work and Jekyll's are too progressive.The next day, Jekyll becomes sullen after learning that the next shipment of live laboratory animals is delayed, thus postponing any trial of the antidote. Although Smithers senses that Jekyll is becoming desperate, he once again avoids telling his master the possibility of his having made a mistake in copying down the antidote. Alone that night, Jekyll tests the formula on himself and is immediately tortured by crushing pain. Looking in the mirror, he sees a pale, gruesome and unrecognizable fiend, Mr. Danny Blood. Despite choking down the antidote, Danny Blood becomes enraged and flees into the streets.Stopping at a pub, Danny Blood grabs singer April Conners, who, despite the strangers gruff manners, eagerly accepts his money for sex. When her latest lover tries to intercede, Blood leads him outside and beats him unconscious with his cane. Unaware of the danger, April takes Blood to her apartment, where he sadistically demands that he serve him, burns her with a cigarette and then beats her mercilessly. After threatening to kill her if she mentions the incident to anyone, Blood returns to the street, where he slashes a prostitute with such precision, that upon finding her dead body, the police suspect the murderer has some medical knowledge.Waking up the next day in his lab, Jekyll cannot recall his evening's activities, but his torn and blood-stained clothes concern his sister Carla (Jacqueline Lawrence). Jekyll, whose temper is now easily aroused, can only show bitter hatred for her and shuts her out of the laboratory. While working with his students that afternoon, Jekyll looses his train of thought as his mind becomes increasingly deranged. Realizing Jekyll's erratic behavior must be the result of not using the correct antidote, Smithers privately admits his mistake to Jekyll, who then explains that after he secretly took the formula and antidote, he was consumed by evil thoughts and now cannot remember his actions from the previous night. Fearing that authorities will shut down his experiments, Jekyll swears Smithers to secrecy until he can find the formula for the real antidote.Returning his students, Jekyll turns vicious and ridicules Victoria, insisting that women were meant to remain home as slaves to men. Jekyll then collapses in a rage, prompting Smithers to sedate the doctor and put him to bed. Fearing for the doctor's health, Smithers tells the details of their research and his own mistake to Carla, who later tells Jekyll that she knows the truth, then leaves to seek help from Oliver. Enraged by Smithers' betrayal, Jekyll transforms into Blood, then murders Smithers and returns to April, whom he beats with a belt. As he leaves her building, Blood pays for another prostitute, who takes him to an orgy, where participants are bound to the walls and tortured.Waking up the next day again with no memory of the past evening, Jekyll conducts his class, then meets Victoria alone. Excited by Jekyll's suggestion for a paper she is writing, Victoria embraces and kisses him. The intimacy prompts his violent urge to return and Jekyll momentarily attacks Victoria. Quickly recovering, Jekyll apologizes, but Victoria is so distraught by his unpredictable behavior that she tells Carla.Meanwhile, April, having found a calling card for Jekyll that fell from Danny Blood's bag, seeks the physicians help, not recognizing him as Blood. Although Jekyll assures her that the man will no longer haunt her, hours later Jekyll again becomes Blood and goes to Aprils apartment, where he beheads her with a hatchet. Meanwhile, Carla, Victoria and Oliver, who have gone to the police, make their way to Jekyll's house to force the doctor to commit himself to an asylum. Having heard the plan, Mary Ann rushes ahead to warn him, but he attacks her. When the group arrives and sees Mary Ann unconscious in the arms of Blood, the policeman shoots and kills Blood, whose body immediately is transformed into Jekyll. Oliver, realizing the truth, cradles the body of his friend.