Gabriel John Utterson and his cousin Richard Enfield are on their weekly walk when they reach the door of a large house located down a by-street in a busy quarter of London. Enfield tells Utterson that months ago, at three o'clock in the morning, he saw a sinister-looking man named Edward Hyde trample a young girl after accidentally bumping into her. Enfield forced Hyde to pay her family £100 to avoid a scandal. Hyde brought Enfield to this door and gave him a cheque signed by a reputable gentleman later revealed to be Doctor Henry Jekyll, Utterson's friend and client. Utterson fears Hyde is blackmailing Jekyll, as Jekyll recently changed his will to make Hyde the sole beneficiary in the event of Jekyll's death or disappearance. When Utterson tries to discuss Hyde with Jekyll, Jekyll says he can get rid of Hyde when he wants and asks him to drop the matter.
A year later in October, a servant sees Hyde beat Sir Danvers Carew, another one of Utterson's clients, to death and leave behind half a broken cane. The police contact Utterson, who leads officers to Hyde's apartment. Hyde has vanished, but they find the other half of the broken cane, which Utterson recognises as one he had given to Jekyll. Utterson visits Jekyll, who produces a note allegedly written to Jekyll by Hyde, apologising for the trouble that he has caused. However, Hyde's handwriting is similar to Jekyll's own, leading Utterson to conclude that Jekyll forged the note to protect Hyde.
For two months, Jekyll reverts to his former sociable manner but, in early January, he starts refusing visitors. Dr Hastie Lanyon, a mutual friend of Jekyll and Utterson, dies of shock after receiving information relating to Jekyll. Before his death, Lanyon gives Utterson a letter to be opened after Jekyll's death or disappearance. In late February, during another walk with Enfield, Utterson starts a conversation with Jekyll at his laboratory window. Jekyll suddenly slams the window shut and disappears, shocking and concerning Utterson.
In early March, Jekyll's butler, Mr Poole, visits Utterson and says Jekyll has secluded himself in his laboratory for weeks. Utterson and Poole break into the laboratory, where they find Hyde's body wearing Jekyll's clothes, apparently having killed himself. They find a letter from Jekyll to Utterson. Utterson reads Lanyon's letter, then Jekyll's.
Lanyon's letter reveals his deterioration resulted from the shock of seeing Hyde drink a serum that turned him into Jekyll. Jekyll's letter explains he held himself to strict moral standards publicly, but indulged in unstated vices and struggled with shame. He found a way to transform himself and thereby indulge his vices without fear of detection. Jekyll's transformed body, Hyde, was evil, self-indulgent, and uncaring to anyone but himself. Initially, Jekyll controlled the transformations with the serum, but one night in August, he became Hyde involuntarily in his sleep.
Jekyll resolved to cease becoming Hyde. Despite this, one night he had a moment of weakness and drank the serum. Hyde, his desires having been caged for so long, killed Carew. Horrified, Jekyll tried more adamantly to stop the transformations. Then, in early January, he transformed involuntarily while awake. Far from his laboratory and hunted by the police as a murderer, Hyde needed help to avoid capture. He wrote to Lanyon in Jekyll's hand, asking his friend to bring chemicals from his laboratory. In Lanyon's presence, Hyde mixed the chemicals, drank the serum, and transformed into Jekyll. The shock of the sight instigated Lanyon's deterioration and death. Meanwhile, Jekyll's involuntary transformations increased in frequency and required ever larger doses of the serum to reverse. It was one of these transformations that caused Jekyll to slam his window shut on Utterson.
Eventually, the supply of salt used in the serum ran low, and subsequent batches prepared from new stocks failed to work. Jekyll speculated that the original ingredient had some impurity that made it work. Realizing that he would stay transformed as Hyde, Jekyll wrote out a full account of the events. Jekyll ends by saying that he does not know if Hyde will be executed or if he will find the courage to commit suicide first, but he does not care, because he (Jekyll) is dying imminently, and whatever happens next happens to someone other than him.
