After her life is upended by divorce and a cross-country move, 16-year-old Saskia Brown feels like an outsider at her new school--not only is she a transplant, but she's also biracial in a population of mostly white students. One day while visiting her only friend at her part-time library job, Saskia encounters a vial of liquid mercury, then touches an old daguerreotype--the precursor of the modern-day photograph--and makes a startling discovery. She is somehow able to visit the man in the portrait: Robert Cornelius, a brilliant young inventor from the nineteenth century. The hitch: she can see him only in her dreams.

Saskia shares her revelation with some classmates, hoping to find connection and friendship among strangers. Under her guidance, the other girls steal portraits of young men from a local college's daguerreotype collection and try the dangerous experiment for themselves. Soon, they each form a bond with their own "Mercury Boy," from an injured Union soldier to a charming pickpocket in New York City.

At night, the girls visit the boys in their dreams. During the day, they hold clandestine meetings of their new secret society. At first, the Mercury Boys Club is a thrilling diversion from their troubled everyday lives, but it's not long before jealousy, violence and secrets threaten everything the girls hold dear.