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It strikes me that it is not desirable to summarize the entire plot of the novel in the intro matter. If anyone wants to add the relevant material back, I am quoting it below.
Jekyll had been trying to invent a potion which could separate his good and evil sides. When Jekyll tested it, he was now transformed into \'Mr Hyde\', a manifestation of his evil side with no trace of morality, but his normal personality remained unchanged. In other words, as Dr. Jekyll he was a man with mostly good and some evil urges, as Mr Hyde he was a man with only evil urges. After some cautious experimenting, Jekyll decided he liked this side-effect. As Mr Hyde, he could indulge himself in every pleasurable vice, and never be suspected as Hyde looked completely different. However, Hyde eventually committed murder, and Jekyll resolved never to use the potion again.
But after a few months, Jekyll began spontaneously changing into Hyde. Only by drinking the potion could he retain his own form, and the potion was running out—not to mention that ever since the murder, the police had been searching relentlessly for Edward Hyde. When Jekyll made a new batch of the potion, it didn\'t work; his original chemical samples had been contaminated, and it was the impurities that had made the transformation possible. At the end of his letter, Jekyll writes that he soon will change into Hyde permanently, and thus his life will end.
While summarizing the entire narrative might be a bit much, the story is so old and well-known that the twist ending is a Late Arrival Spoiler and example of It Was His Sled.
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