The novel begins with "Mr. Utterson the lawyer" going for a walk with his friend and relative Mr. Enfield. They walk past a door, which somehow prompts Mr. Enfield to tell a sad story: a brute of a man knocked down a little girl, everyone yelled at the rude man, the man offered to pay a lot of money and disappeared through the door only to return with a large check drawn from Dr. Jekyll’s bank account. The nasty man? None other than Mr. Hyde.
Mr. Utterson, it turns out, is Dr. Jekyll’s lawyer, and we find out that in the event of Dr. Jekyll’s death or disappearance, his entire estate is to be turned over to Mr. Hyde. Mr. Utterson, who thinks highly of Dr. Jekyll, is extremely suspicious of this whole arrangement. He resolves to get to the bottom of this mystery. He hunts down Mr. Hyde and is suitably impressed with the evil just oozing out of Hyde’s pores. He then asks Dr. Jekyll about these odd arrangements. Dr. Jekyll refuses to comment, and there the matter rests until "nearly a year later."
Cut to "nearly a year later." A prominent politician is brutally beaten to death. The murder is conveniently witnessed by a maid, who points to evil-oozing Mr. Hyde as the culprit. Everyone tries to hunt down this evil man, but with no success. Meanwhile, Dr. Jekyll is in great health and spirits; he entertains his friends (among them one Dr. Lanyon), gives dinner parties, and attends to his religious duties. Two months later, both Dr. Lanyon and Dr. Jekyll fall terribly ill, and claim to have irrevocably quarreled with each other. Dr. Lanyon dies, leaving mysterious documents in Mr. Utterson’s possession, to be opened only if Dr. Jekyll dies or disappears. Dr. Jekyll remains in seclusion, despite frequent visits from Mr. Utterson.
Finally, one evening, Dr. Jekyll’s butler visits Mr. Utterson at home. He’s worried about his master and is convinced of foul play. The butler persuades Mr. Utterson to return to Dr. Jekyll’s house, where they break into Dr. Jekyll’s laboratory. They find Mr. Hyde dead on the floor, with Dr. Jekyll nowhere to be found.
Mr. Utterson finds several documents left to him, and goes back home to read both Mr. Lanyon’s narrative and Dr. Jekyll’s narrative, which, it turns out, are two parts of the same story. Since we’re at the end of the story, author Robert Louis Stevenson figured it was about time to tell us what happened at the beginning. So we discover (through the documents left by the dead men) the following: by means of a potion, Dr. Jekyll was able to transform into Mr. Hyde and give in to a world of pleasure and self-serving crime. In his narrative, Dr. Jekyll writes that Mr. Hyde became ever more powerful and ever harder to control – in essence, the dominant personality.