London 1874. Dr. Henry Jekyll [Paul Massie] is a scientist, rejected by his peers for his research and theories about the human mind. His relationship with his wife Kitty [Dawn Addams] is suffering because he is only interested in his research at his home laboratory. Henry and Dr. Ernst Litauer [David Kossoff] observe mute children playing and fighting in Henry's garden. Henry explains to Ernst that he believes the human mind has a personality only shown in play, only seen when not speaking, and that there are two forces in the mind, fighting for supremacy. Man is two beings, man as he is with moral restrictions and man as he would be free of restrictions. To demonstrate, Henry injects a monkey with a serum to transform its mind and behaviour.Kitty arrives and tells Henry that Paul [Christopher Lee], a socialite with whom she is having an affair, has arrived looking for financial help again. She confides her fears and concerns for her husband with Ernst, saying she is worried about him and his state of mind.That night Kitty is leaving for a social night out. Henry asks her to stay saying he needs her and will take the night off but she refuses and leaves. Henry injects himself with the serum and transforms into a handsome, well dressed young man, Mr. Edward Hyde.At a club, Paul and Kitty dance together. Henry/Edward arrives by cab. He is quickly approached by a girl who takes a fancy to him and they start dancing. Seeing Paul and Kitty, Edward breaks away and introduces himself to them as a friend of Henry Jekyll. After dancing with Kitty, the disgruntled girl he was originally dancing with and her male companion challenge Edward and a fight breaks out, but then Jekyll's and Hyde's personalities start arguing with each other and he quickly leaves the club.Back in his laboratory he transforms fully back into Dr. Jekyll. Henry talks with Kitty about his struggles with his identity and not knowing who he really is.Next night as Edward Hyde he joins Paul and a couple of girls at the club again. They watch Maria [Norma Marla], a belly dancer, dance with a snake. Edward is entranced and after the dance Paul introduces Edward to her. Edward follows her to her room and seduces her. As his personality starts to change back again, Maria mentions his cold wife, putting an idea in his head.Edward visits Kitty and they talk about her relationships with Henry and Paul. He tries to seduce her but she is too much in love with Paul and resists him. As he goes to leave, he starts to transform back to Dr. Jekyll again and rushes back to his laboratory.Ernst visits Henry to check on his health. Paul arrives looking for more money but Henry refuses to give him any saying he is going away, so Paul leaves again saying to Kitty it is over between them.Edward meets Paul at a club and Paul confides that he has been let down by Henry and Kitty and is £2000 in debt. Edward says he will credit Paul £5000 but he can repay him in kind with favours rather than money. They go to a boxing match, go drinking, and smoke opium together and soon the £5000 is gone. Edward agrees to lend him more money in return for Kitty but Paul refuses.Edward visits Kitty again and suggests she buy back Paul's debts by giving herself to him, but she refuses again so he leaves in anger.While Edward frequents bars with various women, Paul and Kitty get back together. As Edward leaves a bar he is mugged and when he regains consciousness he has transformed back into Dr. Jekyll. He finds a note in his pocket from Kitty saying she is leaving him. He decides to destroy his drugs and formula but fears Hyde may have too much influence over him and must exorcise him.Henry transforms back into Hyde and invites Paul and Kitty to meet with Henry at the club. Paul meets Edward in Maria's private room at the club while Kitty waits watching the dancers. Edward shows Paul into a side room and locks him in with Maria's snake. He fetches Kitty and forces himself upon her.Edward and Maria meet back at Henry's house. When Kitty wakes up she sees Paul has been killed by the snake. In her anguish she falls off a balcony and through a glass roof onto the dance floor of the club.In bed with Maria, Hyde's personality starts to struggle with Jekyll's again and as he transforms back into Jekyll he strangles Maria.In his laboratory, his personalities continue their conflict; Hyde wants to be free of Jekyll but Jekyll will be blamed for Maria and Paul's deaths, in which case Jekyll must lose the conflict.The police investigate Kitty and Paul's deaths at the club. Henry writes a letter to Ernst asking him to come and help, but Hyde's personality says Ernst will die if he comes. Henry transforms into Hyde, asks a window cleaner into the laboratory, and then shoots him. He splashes oil around the laboratory as the police arrive. He pretends Jekyll is in the laboratory with him with a gun then sets fire to the place and fires a shot. As the police break down the door, Hyde runs out saying Jekyll has shot himself and set fire to the lab.Later at the inquest, the conclusion is that Jekyll committed suicide due to complications of his mind from his experiments. As Hyde and Ernst leave the inquest, Hyde again struggles to contain Jekyll's personality and to the amazement of Ernst and the police officer turns back into Jekyll before them. Jekyll has destroyed Hyde and in doing so has destroyed himself. He is arrested for murder.