Winter 1888. At an asylum in the English countryside, a man suspected of being Jack the Ripper kills an orderly and flees into the rain-soaked night. His distraught keepers summon the Lady Vivienne Cumberland-who's interviewed their patient and isn't sure he's a man at all. An enigmatic woman who guards her own secrets closely, Lady Vivienne knows a creature from the underworld when she sees one. And he's the most dangerous she's ever encountered.As Jack rampages through London, Lady Vivienne begins to suspect what he's searching for. And if he finds it, the doors to purgatory will be thrown wide open...Across the Atlantic, an archaeologist is brutally murdered after a Christmas Eve gala at the American Museum of Natural History. Certain peculiar aspects of the crime attract the interest of the Society for Psychical Research and its newest investigator, Harrison Fearing Pell. Is Dr. Sabelline's death related to his recent dig in Alexandria? Or is the motive something darker?As Harry uncovers troubling connections to a serial murder case she'd believed was definitively solved, two mysteries converge amid the grit and glamor of Gilded Age New York. Harry and Lady Vivienne must join forces to stop an ancient evil. The key is something called the Thirteenth Gate. But where is it? And more importantly, who will find it first?