Goss introduces us to Mary Jekyll, whose well-regarded scientist father died when she was a child. While cleaning up her recently deceased mother’s affairs, she learns of an account in her name supporting someone named Hyde. With the death of her mother, her first priority is to get her household back in order, and to figure out how to pay off old debts. She enlists the services of Sherlock Holmes and Dr. Watson to investigate, believing the person to be a notorious and brutal associate of her father’s, Edward Hyde, who is wanted for murder. Mary hopes the money from a long-offered reward would help set her house in order. Instead of the wanted criminal, she discovers that the money is supporting a feisty young woman named Diana Hyde, left in the care of a charitable organization.
Intrigued, she discovers that her late father was part of a secretive organization known as the Société des Alchimistes, whose members include scientists such as Victor Frankenstein, Doctor Moreau, Abraham Van Helsing, and Giacomo Rappaccini, all of whom seem to be working toward the goal of transforming humanity. As she searches, she meets and befriends the daughters of these male scientists, all of whom have been subjected to experiments that have transformed them. There’s Catherine “Cat” Moreau, who was a puma transformed through a series of experiments into a human; Beatrice Rappaccini, a woman who breathes poison; and Justine Frankenstein, a woman reanimated as a lover for Frankenstein’s original monster.