Summary:
During their weekly walk, an eminently sensible, trustworthy lawyer called Mr. Utterson listens as his friend Enfield recounts the gruesome tale of the assault. The tale describes a sinister figure named Mr. Hyde tramples a young girl and thereafter disappears into a street door, then re-emerges to pay off her relatives with a check signed by a respectable gentleman. Since both Utterson and Enfield disapprove of gossip, they agree to speak no further about the matter. However, it happens that one of Utterson’s clients and close friends, Dr. Jekyll has written a will transferring all of his property to this same Mr. Hyde. Soon, Utterson begins having dreams in which a faceless figure stalks the streets through a nightmarish version of London.

Puzzled, the lawyer visits Jekyll and their mutual friend Dr. Lanyon to try to learn more. Lanyon reports that he no longer sees much of Jekyll, since they had a dispute over the course of Jekyll’s research, which Lanyon labels “unscientific balderdash.” Curious, Utterson stakes out a building that Hyde visits—which, it turns out, is a laboratory attached to the rear of Jekyll’s home. Encountering Hyde, Utterson is quite taken back, by how undefinably ugly the man seems, as if deformed, even if Utterson cannot say exactly how. Much to Utterson’s surprise, Hyde willingly offers Utterson his address. Jekyll tells Utterson not to concern himself with the matter of Hyde.
A year passes by uneventfully. Then, one night, a servant girl witnesses Hyde brutally beat to death an old man named Sir Danvers Carew, a member of Parliament and a client of Utterson. The police contact Utterson, and Utterson suspects that Hyde is the murderer. He leads the officers to Hyde’s flat, feeling a sense of foreboding amid the eerie weather—the morning is dark and wreathed in fog. When they arrive at the flat, the murderer has vanished, and police searches prove futile. Shortly thereafter, Utterson again visits Jekyll, who now claims to have ended all relations with Hyde; he shows Utterson a note, allegedly written to Jekyll by Hyde, apologising for the trouble he has caused him and saying goodbye. That night, however, Utterson’s clerk points out that Hyde’s handwriting bears a remarkable similarity to taht of  Jekyll’s.
For a few months, Jekyll acts especially friendly and sociable, as if a weight has been lifted from his shoulders. But then Jekyll suddenly begins to refuse visitors, and Lanyon dies from some kind of shock he received in connection with Jekyll. Before dying, however, Lanyon gives Utterson a letter, with instructions that he not open it until after Jekyll’s death. Meanwhile, Utterson goes out walking with Enfield, and they see Jekyll at a window of his laboratory; the three men begin to converse, but a look of horror comes over Jekyll’s face, and he slams the window and disappears. Soon afterward, Jekyll’s butler, Mr. Poole, visits Utterson he is in  a state of desperation: Jekyll has isolated himself in his laboratory for several weeks, and now the voice that comes from the other side of the door, sounds nothing like the doctor’s. Utterson and Poole travel to Jekyll’s house through empty, windswept, sinister streets; once there, they find the servants huddled together in fear. After discussing for a while, the two decide to break into Jekyll’s laboratory. Inside, they find the body of Hyde, wearing Jekyll’s clothes and apparently dead by suicide—and a letter from Jekyll to Utterson promising to explain everything.

Utterson takes the document home, where first he reads Lanyon’s letter; it reveals that Lanyon’s deterioration and eventual death were caused by the shock of seeing Mr. Hyde drink a potion and there after metamorphose into Dr. Jekyll. The second letter is a testament by Jekyll. It explains how Jekyll, seeking to separate his good side from his darker impulses, discovered a way to transform himself periodically into a deformed monster free of conscience—Mr. Hyde. At first, Jekyll recounts, he is delighted in becoming Hyde and rejoiced in the moral freedom that the creature possessed. Eventually, however, he found that he was involuntarily turning into Hyde during his sleep, even without taking the potion. At this point, Jekyll decided to cease becoming Hyde. One night, however, the urge possesed him too strongly, and after the transformation he immediately rushed out and violently killed Sir Danvers Carew. Horrified, Jekyll tried very hard to stop the transformations, and for a limited time he proved successful; one day, however, while sitting in a park, he suddenly turned into Hyde, this was the first time that an involuntary metamorphosis had happened while he was awake.

The letter continues describing Jekyll’s cry for help. Distant from his laboratory and hunted by the police as a murderer, Hyde needed Lanyon’s help to get his potions and return back to Jekyll again, but when he undertook the transformation in the presence of Lanyon, the shock of the sight killed Lanyon. Meanwhile, Jekyll returned home, only to find himself ever more helpless and trapped as the transformations increased in frequency and rrequired even larger doses of potion in order to reverse the effects. It was the onset of one of these spontaneous metamorphoses that caused Jekyll to slam his laboratory window shut in the middle of his conversation with Enfield and Utterson. Eventually, the potion began to run out, and Jekyll was unable to find a key ingredient to make more. His ability to change back from Hyde into Jekyll slowly vanished. Jekyll writes that even as he writes the  letter he knows that he will soon become Hyde permanently, and he wonders if Hyde will face execution for his crimes he has commited or choose to kill himself. Jekyll notes that, in any case, the end of his letter marks the end of the life of Dr. Jekyll. With these words, both the document and the novel come to a close.