Psychiatrist Dr Marlowe enters his gentlemen's club, where Lanyon, Utterson and Enfield are discussing the nature of evil. Marlowe voices his opinion that all humans have two sides, good and evil, and that they can be separated.

In his lab, Marlowe injects his cat with a drug he has concocted; the cat attacks him and he kills it with a poker. A female patient arrives unexpectedly for a session. When Marlowe leaves the room, she opens a drawer marked 'Personal' and finds photographs of naked women inside. Marlowe injects her with his drug and she tries to seduce him. Later, a male patient comes to see Marlowe; he's furious at the failure of his treatment. When Marlowe injects him, he becomes grovelling and wretched.

Marlowe recounts the results of his experiments and Lanyon asks if it has the same effect on a person more than once. Marlowe decides to inject himself to find out. As the drug takes effect, he becomes ugly and almost kills a lab mouse. When he injects himself with the antidote, he changes back.

Marlowe and Lanyon discuss Freud's three parts of a person: the id, the ego and the superego. They deduce that the drug destroys all but one part and that part differs in different people. Lanyon argues that if the superego was destroyed, the person would become a monster.

Marlowe injects himself again and goes out, starting a fight and stealing a cane from a shop. He rents a room in a rundown chamber.

Back at the club, the men discuss the spate of seemingly motiveless crimes. Enfield describes such a crime to Utterson: he saw a man trample a little girl in the street, chased the man and brought him back, insisting that he pay the child's family compensation. The man went into Marlowe's house and returned with a cheque signed by him. The man who trampled the child called himself Edward Blake.

Utterson goes to see Lanyon who explains that he has fallen out with Marlowe and that he knows nothing of an Edward Blake.

Marlowe injects himself again and goes out into the street. Utterson wakes from a nightmare and goes to Marlowe's house, but is told he's out. He learns that everyone in the house has orders to obey Blake. Suspecting that Blake is a blackmailer, Utterson watches Marlowe's house. He confronts Blake, who hits him with his cane.

Utterson tells Marlowe that he should rid himself of Blake. Marlowe explains that Blake has discovered the pleasures of evil.

Marlowe injects himself again and goes to a tavern. He buys a woman a drink but she taunts him, calling him ugly, provoking laughter among the drinkers. Blake leaves but later follows the woman to the docks. Cornering her, he beats her to death with his cane, leaving the stick behind as he flees. Back in his room, he contemplates his reflection, horrified.

Utterson reads of the murder. Recognising the cane as Blake's, he calls on Marlowe and tells him that he suspects Blake. Marlowe tells him that Blake has left the country, showing Utterson a letter from him. But Utterson is suspicious and ascertains that no letter was delivered to the house. Later, Utterson notices the similarity between the handwriting on Blake's letter and Marlowe's own writing and returns to Marlowe's house to wait for him.

Marlowe begins to turn into Blake while in the park, so he returns home. Losing the key, he approaches the front door but, hearing Utterson inside, goes instead to his chambers.

Lanyon receives a note from Marlowe, asking for help. He fetches the drugs and syringe and returns home. Blake bursts in and grabs the antidote but Lanyon won't let him go. Blake injects the antidote and changes back into Marlowe before his eyes; Lanyon has a heart attack and dies.

Back at his lab, Marlowe again begins to change. He breaks into Utterson's house and the two men fight, knocking over a lamp which starts a fire. Blake catches fire then falls down the stairs. Utterson runs onto the landing to see Blake turn back into Marlowe.