Miss Ada Quicksilver, an orphan with a small inheritance and an inquiring mind, is a student of London's Lovelace Academy for Promising Young Women. Pursuing her anthropological study of the fairies' disappearance, she spends her Christmas holiday in Ireland.
In Dublin's absinthe bars, she investigates a supposed association between the bittersweet spirit and fairy sightings. One night in the Green Fairy, a handsome and intriguing Irishman approaches her, introducing himself as Edward Donoghue. He takes absinthe regularly to relieve a mysterious affliction. She soon discovers that "Mr. Donoghue" is the twelfth earl of Meath and that in two days, he is to visit the Newgrange passage tomb, which has recently come under the protection of his earldom. On hearing of Ada's research, he invites her to accompany him. Separate lodgings, of course—the earl is nothing if not a gentleman.
They arrive in time for the winter solstice, when the first rays of dawn stream directly down the passage into the main cavern. That night, the secret of Meath's affliction is revealed: he is, in fact, a lord in two worlds and can no longer suppress his shadow self.
Ada is torn between a sensible fear of becoming entangled with this troubled gentleman, and her compelling desire to ease his suffering. She finally accepts his invitation to accompany him farther, to the winter retreat of his cousin, Ireland's beautiful, half-mad Queen Isolde.
Little does either of them realize that their blossoming friendship—and slowly kindling passion—will lead to discoveries that wrench open a door sealed for centuries, throwing them into a war that will change Ireland forever.