Mr. Utterson is a lawyer. He is reserved but kind and is known for loyally sticking by his friends even when they do wrong. One of his unlikely friends is Mr. Enfield. One Sunday, as the pair is taking a walk, they come across a somber looking door belonging to a house that Enfield knows well. He tells the story of a horrible incident, in which a man trampled a young girl and, when apprehended, seemed remorseless but agreed to pay a large check when threatened by the police. He disappeared into this very house and revealed a check signed by a well-known and respected name.
Mr. Utterson and Mr. Enfield agree that it is best not to talk any further about the matter but Utterson is deeply affected, because he knows the fiend that Enfield describes. One of his clients, Dr. Jekyll, has recently made a will and has left everything to a Mr. Hyde, rather than his own family. He visits Dr. Lanyon, an old friend of Jekyll's who has had a falling out with Jekyll over what he considers to be his old friend's unscientific methods. Lanyon has never heard of Mr. Hyde, causing Utterson to worry even more about Jekyll’s safety. He has nightmares of Jekyll being woken in his bed by this blackmailing fiend.
So, Utterson decides to spy on the strange house, the scene of the crime. Finally one night, he sees Hyde approach and confronts him and senses the same air of evil about the man that Enfield described. He goes to Jekyll’s house and, finding Jekyll absent, asks Poole, a servant, about Mr. Hyde. Poole has been instructed to treat Mr. Hyde almost like a master, continuing Utterson’s anxiety. Soon after, at one of Jekyll’s customary dinner parties, Utterson stays behind and asks his friend what the matter is, but Jekyll will not confess, he only cryptically says that he can choose to be free of Hyde whenever he likes.
A year passes, and again Hyde is involved in a horrific crime, this time the murder of a respected old man named Sir Danvers Carew. Because Sir Carew was a client of Utterson’s, the police come to him, and Utterson takes them to Hyde’s address, but they find nothing amiss in his rooms, only a burnt-out end of a checkbook. Utterson visits Jekyll, who claims that he is finally finished with Mr. Hyde; he even shows Utterson a letter from Hyde to the same effect. But when Utterson goes home, and sits with his trusted clerk, a handwriting expert, the letter turns out to be written in Jekyll’s own hand instead. This brings back Utterson’s suspicions of blackmail.
After the horrific murder of Carew has become public news, Jekyll is back to his old self and regularly entertains his friends. But after two months, Utterson is turned away from Jekyll’s door and once again the doctor becomes a recluse. When Utterson goes to ask Dr. Lanyon about it, Lanyon is a changed man – he says he has had a shock that will soon kill him. Though he will not say the nature of what happened between him and Jekyll, Lanyon gives Utterson a letter to read when Jekyll is dead.
Utterson, after a while, pays Jekyll fewer and fewer visits, but one day, when out walking with Enfield, they pass his old lab (which the "somber" door leads into) and decide to go through the yard and say hello at the window. Jekyll greets them but is overtaken by a strange mood and disappears from the window. A few weeks later, Poole visits Utterson in a panic and persuades him to come to the house, where Utterson finds all the servants cowering in fear of their changed master. Jekyll has locked himself away and the voice that comes from his cabinet (the upper room of his laboratory) is not his but Mr. Hyde’s. Utterson and Poole, thinking Jekyll has been murdered, break in to the cabinet. They find Hyde’s dead body on the floor and some documents, including a letter from Jekyll, saying that it is time for him to reveal the truth.
Utterson takes these documents home to read. First, he reads the narrative given to him by Dr. Lanyon. He explains the shock that has taken his life. He was asked by Jekyll to fetch him a drawer of ingredients from Jekyll's lab for an experiment and to await a visit from a man. This, Lanyon did, very curious now about Jekyll’s secret. When the visitor arrived—who from Lanyon's description is clearly Mr. Hyde—he makes a potion and drinks it. His body begins to warp. When this horrific display is done, Dr. Jekyll is standing before Lanyon.
Next, Utterson reads Dr. Jekyll’s own confession. Jekyll describes his theory that all human beings have two natural selves, one good and one evil. He has felt this way all his life and has now succeeded in finding a way to separate the two. He describes from his own point of view all the events that his friends have witnessed. At first, the ability to become Mr. Hyde gives Jekyll a freeing new life in which he can indulge his basest instincts, but soon Mr. Hyde begins to do unspeakable things, such as murder Carew. Jekyll decides to cease transforming into Hyde, but one day, in a park, Jekyll turns into Hyde involuntarily—without taking the potion. That is when he must confess to Lanyon to procure the chemicals he needs to transform back.
Jekyll returns home but again Hyde takes over, now actively resentful of Jekyll, and Jekyll is forced to lock himself in his lab and send Poole out for more chemicals. But the potion has lost its effectiveness and as he writes the last of his confession, he is using the last of the original powders and anticipates turning finally into Mr. Hyde forever. No longer inspired by his belief in a double nature, he believes that this moment will be a complete end to him, and that Hyde will go on as a separate being, left to deal with his new undivided condition.