The story follows the dual nature of Dr. Jekyll, a respected scientist who creates a potion that transforms him into the evil Mr. Hyde. The two personalities, one representing good and the other representing evil, become increasingly intertwined, ultimately leading to tragedy.
The novella's immense popularity quickly established the term "Jekyll and Hyde" as a common phrase to describe someone who exhibits a dramatic and unsettling shift in personality, often between a seemingly respectable persona and a hidden, darker one.