Mary Reilly is a retelling of Dr. Jekyll and Mr. Hyde through the eyes of a servant named Mary Reilly. Set in Victorian England, she serves at Dr. Jekyll's manor and grows devoted to him as he struggles with his sinister alter-ego, Mr. Hyde. Dr. Jekyll created Mr. Hyde through various experiments in his laboratory. In the beginning, Dr. Jekyll exhibits interest in Mary beyond any other servant. For instance, he inquires about the scars on her hands and neck and asks her to write a letter explaining how she received them.

The story opens with her letter, revealing that her abusive father locked her beneath the stairs and released a rat that bit her repeatedly. Eventually, Mary keeps his secrets and visits his lab. Throughout the story, Hyde has committed numerous crimes, including murder.

As Hyde gets into trouble, Dr. Jekyll relies on her as a confident, with some sexual tension hovering between the two. Despite Hyde's crimes, Mary senses that Dr. Jekyll is her equal due to the sadness within them, but the class differences between the two proved to be an invisible barrier. In addition, she chose not to leave his side after Hyde threatened her at the end of the story. After Dr. Jekyll committed suicide, she still exhibited devotion to him.