There are winged horses that live in the mirrors of Briar Hill hospital. In the mirrors that line its grand hallways, which once belonged to a princess. In those that reflect the elegant rooms, now filled with sick children. It is her secret.

One morning, when Emmaline climbs over the wall of the hospital’s abandoned gardens, she discovers something incredible: a white horse with broken wings has left the mirror-world and entered her own.

Tucked into the garden’s once-gleaming sundial, Emmaline finds a letter from the Horse Lord. He is hiding the wounded white horse, named Foxfire, from a dark and sinister force—a Black Horse who hunts by colorless moonlight. If Emmaline is to keep the Black Horse from finding her new friend, she must collect colorful objects with which to blind him. But where can Emmaline find color when her world is filled with gray?