It’s 1926. When 17-year-old Evie’s gin-induced shenanigans threaten to disgrace the family name, her parents send her to stay with Uncle Will in New York. Evie is secretly delighted. She can’t wait to take in the bustling city with its jazz, speakeasies and shows. Her pen pal, Mabel, lives in Uncle Will’s building, so Evie will have a partner in crime. She hopes the new surroundings will distract her from her vivid nightmares and her strange, disturbing ability to read people’s histories by touching their personal items.
When Evie arrives in New York, she’s wooed and pick-pocketed by a handsome young man named Sam Lloyd. She learns her uncle’s business, The Museum of American Folklore, Superstition and the Occult, is facing bankruptcy. Sam joins the small museum staff. Evie tries to help Uncle Will and his assistant, Jericho, drum up publicity. When the police ask Will to lend his insight into a string of gory ritual murders, Evie, Sam and Jericho help out. Evie uses her supernatural abilities to get information from the deceased victims.
Meanwhile, 17-year-old Memphis Campbell runs numbers for the superstitious residents of Harlem. Since the death of their mother and abandonment by their father, Memphis and his beloved younger brother, Isaiah, live with their Aunt Octavia. Memphis dreams of making his mark in the world as a writer, and he often does his writing in a cemetery. He also has nightmares as terrifying and cryptic as Evie’s. Memphis emerged from an illness at age 14 with the ability to heal people.
Evie becomes friends with Theta, a Ziegfeld dancer in her building. Theta lives with a gay piano player named Henry. Both Theta and Henry, as well as Sam and Isaiah, have unique supernatural gifts like Memphis’ and Evie’s. Will tells Evie about the prophesies of a long-dead woman who said people with powers, called Diviners, would eventually be needed to save and protect the world.
Evie and Will believe the serial murderer the police are tracking is actually the partial reincarnation of a man named John Hobbes. Decades earlier, he was a member of a cult called the Brethren. Evie and Will are convinced he is gradually being resurrected in conjunction with the arrival of a comet.
The Brethren believed in a text called The Book of the Holy Brethren, which includes a mixture of references from “Revelation” and biblical-sounding but occult-based teachings. Hobbes repeatedly states he is the Great Beast, the Dragon of Old. Each of his killings is based on one of 11 sacrificial rituals from The Book of the Holy Brethren.
When the comet gives him full bodily form, he intends to bring the end times and cleanse the world. Hobbes ingests the body parts of his victims to become stronger. Since he leaves the mark of a five-pointed star (or Pentacle) and a snake eating its tail on his victims, the press nicknames him the Pentacle Killer.
Evie investigates a run-down mansion called Knowles End. Its original owner was a woman named Ida. When young, she became enamored with spiritualism after the death of her parents. She invited many card readers and mediums to visit, but became close with one named Mary White and her partner, John Hobbes. Eventually, Mary and Hobbes bought the place from Ida, still allowing her to live there. They began to poison her slowly while they performed drug-induced ceremonies for strangers in the rooms below.
Theta and Memphis develop a romance while Evie and Jericho grow closer by working together on the case. Jericho eventually admits he is a human hybrid, rebuilt by the military after he suffered serious war injuries. The Pentacle Killer murders Memphis’ best friend and unsuccessfully targets Theta.
Evie, Will and Jericho sneak into the town where Hobbes is buried. They dig him up and take his pendant, planning to get rid of his body. When Jericho and Evie go to look for Hobbes at Knowles End, the killer captures them. The house comes supernaturally alive at Hobbes’ command. In the nick of time, Evie chants the phrases that suck Hobbes’ spirit into his pendant, supposedly destroying him permanently.
Will agrees to let Evie stay with him in New York. Ominous events and dreams indicate the Diviners will soon be called upon to fight more evil in the world.