When her mother’s new boyfriend turns violent, Sloane Riley finds herself in danger. The seventeen year old is sent to a boarding school in Ireland, leaving behind her four-year old brother, Seth, in London with her mother. There, Sloane meets Nodle, a young groundskeeper. Sloane confesses that she had an imaginary playmate by that name when she was eight, and is surprised when Nodle shares that he had an imaginary friend too—a little girl. Even more mystifying, her new roommate, Kelia, is a famous model who seems indifferent to her own beauty and success. Kelia is an unpretentious beauty who shuns attention and fame, preferring to live simply and walk alone in the woods.

On the night of the fall equinox, Sloane follows Kelia into the forest to a forbidden stone ruin, but finds instead a door to the world of Faerie. Just on the other side, she meets Lythrum, the magnificent Prince of Trees who tells her he has been waiting for her. With this stunning discovery, Sloane begins to understand that there are links between herself and this utterly new, yet strangely familiar, world. But before she can even begin to grasp what’s happening, her brother Seth is taken into Faerie by the Gray King of the underworld.

With the help of a pixie called Auntie, a changeling named Nemus, and none other that the impish Puck himself, she sets out to do what no other mortal could—face the Gray King and fight to keep the door open between the two worlds. It’s up to Sloane to rescue her friends and help Kelia restore the balance of nature, the seasons’ four crowns, and the knowledge that everything, in both worlds, is elementally connected.