Every Sunday, Mr. Utterson, a prominent London lawyer, and his distant kinsman, Mr. Richard Enfield, take a stroll through the city of London. Even though to a stranger's eyes, these two gentlemen seem to be complete opposites, both look forward to, and enjoy, their weekly stroll with one another.
One Sunday, they pass a certain house with a door unlike those in the rest of the neighborhood. The door reminds Mr. Enfield of a previous incident in which he witnessed an extremely unpleasant man trampling upon a small, screaming girl while the strange man was in flight from something, or to somewhere. The screams from the small girl brought a large crowd, and various bystanders became incensed with the indifference of the stranger, whose name they discovered to be Mr. Edward Hyde. Enfield can recall the man only with extreme distaste and utter revulsion. The crowd forced the man to make retribution in the form of money, and they were all surprised when he returned from inside the "strange door" with ten pounds in gold and a check for ninety pounds. They held him until the banks opened to make certain that the check was valid because it was signed by the well-known Dr. Henry Jekyll, and they suspected that it was a forgery. To their amazement, the check was valid.
That evening, in his apartment, Mr. Utterson has further reason to be interested in Mr. Hyde because Dr. Jekyll's will has an unusual clause that stipulates that Edward Hyde is to be the sole beneficiary of all of Jekyll's wealth and property. Utterson goes, therefore, to visit an old friend, Dr. Lanyon, who tells him that some ten years ago, he and Dr. Jekyll became estranged because of a professional matter. Utterson decides to seek out Hyde, and he posts himself as a sentinel outside the mysterious door previously mentioned by Enfield. After some time, Utterson encounters the man Hyde entering the door, and he initiates a conversation with him. Hyde suddenly becomes highly suspicious of Utterson's interest in him and quickly retreats inside the door. Utterson walks around the block and knocks at the front door of Dr. Jekyll's house. Upon questioning the butler, Poole, Utterson discovers that Edward Hyde has complete access to Jekyll's house.
About a fortnight later, Utterson is invited to one of Jekyll's dinner parties and remains after the other guests have left so that he can question Jekyll about his will and about his beneficiary, Edward Hyde. Jekyll is unhappy discussing Edward Hyde and insists that his wishes — that Mr. Hyde be the recipient of his property — be honored.
About a year later, an upstairs maid witnesses the vicious murder of a kindly and distinguished old gentleman, the prominent Sir Danvers Carew, M.P. (Member of Parliament). But the assailant escapes before he can be apprehended. The maid, however, is able to positively identify the murderer as Edward Hyde. Mr. Utterson and the police go to Hyde's apartment, but the housekeeper informs them that he is gone. When Utterson confronts Jekyll about the whereabouts of Hyde, Jekyll shows the lawyer a letter which Hyde wrote saying that he was disappearing forever. Jekyll maintains that he himself is completely through with him.
After the disappearance of Hyde, Jekyll comes out of his seclusion and begins a new life, for a time. But at about the same time, Utterson is dining with his friend, Dr. Lanyon, and he notes that Dr. Lanyon seems to be on the verge of a complete physical collapse; Lanyon dies three weeks later. Among his papers is an envelope addressed to Utterson, and inside is an inner envelope, sealed with instructions that this envelope should not be opened until after Jekyll's death or disappearance. Utterson strongly feels that the contents of the envelope contain information about Edward Hyde.
On another Sunday walk, Utterson and Enfield pass along the street where Enfield saw Hyde trampling on the young girl. They step around the corner into the courtyard and see Dr. Jekyll in an upstairs window. Utterson invites Jekyll to accompany them on a walk, but suddenly Jekyll's face is covered with abject terror and, after a grimace of horrible pain, he suddenly closes the window and disappears. Utterson and Enfield are horrified by what they have seen.
Some time later, Utterson receives a visit from Poole, Dr. Jekyll's man servant. Poole suspects that foul play is associated with his employer; Dr. Jekyll, he says, has confined himself to his laboratory for over a week, has ordered all of his meals to be sent in, and has sent Poole on frantic searches to various chemists for a mysterious drug. Poole is now convinced that his employer has been murdered and that the murderer is still hiding in Jekyll's laboratory.
Utterson is sufficiently convinced that he returns to Jekyll's house, where he and Poole break into the laboratory. There, they discover that the mysterious figure in the laboratory has just committed suicide by drinking a vial of poison. The body is that of Edward Hyde. They search the entire building for signs of Jekyll and can find nothing, except a note addressed to Utterson.
The note informs Utterson that he should go home and read, first, the letter from Dr. Lanyon and then the enclosed document, which is the "confession" of Dr. Henry Jekyll.
Dr. Lanyon's narrative reveals that Dr. Jekyll had written to him, in the name of their old friendship, and had requested him to follow precise instructions: go to Jekyll's laboratory, secure certain items, bring them back to his house, and at twelve o'clock that night, a person whom Lanyon would not recognize would call for these things. Lanyon writes that he followed the instructions precisely and at exactly twelve o'clock, a horribly disagreeable, misbegotten "creature" appeared at the laboratory to claim the items for Dr. Jekyll. Before leaving, he asked for a "graduated glass," proceeded to mix the powders and liquids, and then drank the potion. To Dr. Lanyon's horror, the figure transformed before his eyes into that of Dr. Henry Jekyll. Lanyon closes his letter by pointing out that the man who stepped into the house that night to claim Jekyll's items was the man known as Edward Hyde.
The final Chapter gives a fully detailed narration of Dr. Jekyll's double life. Jekyll had been born wealthy and had grown up handsome, honorable, and distinguished, and yet, he committed secret acts of which he was thoroughly ashamed; intellectually, he evaluated the differences between his private life and his public life and, ultimately, he became obsessed with the idea that at least two different entities, or perhaps even more, occupy a person's body. His reflections and his scientific knowledge led him to contemplate the possibility of scientifically isolating these two separate components. With this in mind, he began to experiment with various chemical combinations. Having ultimately compounded a certain mixture, he then drank it, and his body, under great pain, was transformed into an ugly, repugnant, repulsive "being," representing the "pure evil" that existed within him. Afterward, by drinking the same potion, he could then be transformed back into his original self.
His evil self became Edward Hyde, and in this disguise, he was able to practice whatever shameful depravities he wished, without feeling the shame that Dr. Jekyll would feel. Recognizing his two "selves," Jekyll felt the need of providing for, and protecting, Edward Hyde. Therefore, he furnished a house in Soho, hired a discreet and unscrupulous housekeeper, and announced to his servants that Mr. Hyde was to have full access and liberty of Jekyll's residence and, finally, he drew up a will leaving all of his inheritance to Edward Hyde. Thus, this double life continued until the murder of Sir Danvers Carew by Edward Hyde.
This horrible revelation caused Jekyll to make a serious attempt to cast off his evil side — that is, Edward Hyde — and for some time, he sought out the companionship of his old friends. However, the Edward Hyde side of his nature kept struggling to be recognized, and one sunny day while sitting in Regent's Park, he was suddenly transformed into Edward Hyde. It was at this time that he sought the help of his friend Dr. Lanyon. He hid in a hotel and wrote a letter asking Dr. Lanyon to go to the laboratory in his house and fetch certain drugs to Lanyon's house. There, Hyde drank the potion described in Lanyon's letter. The drug caused him to change to Dr. Jekyll, while Dr. Lanyon watched the transformation in utter horror.
After awhile, Edward Hyde almost totally occupied Jekyll's nature, and the original drug was no longer effective to return Hyde to Jekyll. After having Poole search throughout London for the necessary "powder," Jekyll realized that his original compound must have possessed some impurity which cannot now be duplicated. In despair at being forced to live the rest of his life as Hyde, he commits suicide at the moment that Utterson and Poole are breaking down the laboratory door.