The story is told through the eyes of a man of reason. He is lawyer named Utterson; and, at the beginning of the novella, he is worried for his friend, Henry Jekyll--a violent and odious man who is living in Jekyll's house. Despite Utterson's attempts to speak to his friend, Jekyll completely avoids the subject.
In the later part of the novel, Jekyll locks himself in his laboratory. Utterson is afraid for his friend, so he breaks down the door to save him. Upon entering the laboratory, he finds Hyde--killed by his own hand. A letter--made out in the hand of Jekyll--explains that the doctor had created a potion that divided the two sides of a man's character--the good in him from the bad--allowing a man to transform between the two. Hyde was the manifestation of Jekyll's evil side.
At first Jekyll found the transformation liberating: he was able to live out a life of licentiousness and sin without any danger. However, he soon found that Hyde was taking over. Jekyll was unable to control the process of transformation, and he was certain that he would become Hyde forever, so he killed himself.
Despite using a number of the aspects of the Gothic tradition that had become popular in the nineteenth century through writers such as Mary Shelley and Bram Stoker, Robert Louis Stevenson gives his "monster" a thoroughly modern twist. Although a reader of Jekyll and Hyde is led to believe that the threat of evil comes--as in earlier Gothic stories--from without, it is actually within the breast of the good and kind Jekyll that the danger lurks. The evil within him eventually leads to his destruction. Weaving in the aspects of science with the sudden revelatory twist that comes in the form of a letter from Jekyll's own hand, Robert Louise Stevenson confounds readers' expectations and forces them to reconsider the meaning of what it is to be evil.
What's more, The Mysterious Case of Dr. Jekyll and Mr. Hyde (which is written to a certain extent like a scientific investigation on the part of Utterson into his friends strange behavior) provides a skeptical view of scientific advances, and the limits of rationality. Taking up a thread from classical myth, Stevenson presents Jekyll as an Icarus-like figure who strives to become great and, in doing so, falls to the lowest depths of inhumanity. He is the image--not unlike that of Dr. Frankenstein (from Mary Shelley's novel)--of the man of science who, in attempting to discover more about the world and its inhabitants, becomes a prisoner of his own accomplishment.
More than anything, however, Jekyll and Hyde works as an allegorical portrayal of the goodness and evil that resides in equal measure within the soul of a man. It pre-empted Freudian psychoanalysis (which really only began to be common currency on the publication of Freud's Interpretation of Dreams in 1901) by twenty-five years, and yet is eerily similar to some of its theories.
Under the constraints of rigid Victorian society, the unprepossessing Jekyll learns to give into his inner desires (the instinctive forces Freud termed the Id) when he is transformed into Hyde. The rational, controlled, civilised part of Jekyll attempts to (like Freud's super-ego) repress the Id, and make Hyde controllable. However, as Freud pointed out in his studies of neurotic patients, such a repression of the driving force of nature within us often leads to horrible, barbaric consequences.
Both an exciting horror story, and a perceptive allegorical portrayal of what it is to be human, The Mysterious Case of Dr. Jekyll and Mr. Hyde, works on its readers on a number of levels. Insightful, well-written, and extremely enjoyable, it is a story that chimes so with our collective consciousness, that it has entered into legend.