In present-day Tralee, Ireland, LIAM FARRELL and his twin sister EVELYN "EVE" FARRELL are celebrating their seventeenth birthday when a drive-by shooter fatally wounds Eve. When she dies in Liam's arms, her soul enters his mind.
After Eve's funeral, Liam hears her voice in his head and fears he's going insane. Eve tells him that her soul is inside him, and proves it by borrowing his body while he sleeps to write a letter to him. Liam keeps Eve a secret from their parents, and starts researching anything he can find on magic and the dead. Nothing in the public library is any use. A handwritten note bearing two names, an address, and instructions on how to get there proves far more useful.
Liam convinces his parents to leave town, and proceeds to follow the note's instructions by getting lost in a forest south of Tralee. When he grows doubtful and tries to find his way back, an unseen figure pursues him. Liam is wounded and falls unconscious. He wakes in the care of the very people he had sought: an enchanter known only as MIST, and his partner, a necromancer named MITCHELL HAYES-KAVANAGH. They live in the Everywood, a magically-created realm with gates to every forest in every place and time on Earth. Their cottage is warded against intruders, from wild animals to other mages.
Liam asks the two mages for help in getting Eve's soul out of his head. Mitchell agrees to do his best to help, with no guarantees of safety or success, as tampering with souls is difficult and dangerous. He also warns Liam about the Order of the Shroud, a sect of amoral necromancers, whom he believes is targeting the Farrell twins for their own purposes. Mitchell suspects they had a hand in Eve's death, but has not yet discovered why.
As weeks pass, Eve grows impatient with Mitchell's slow progress in finding a way to free her. Liam discovers she has been borrowing his body at night without his permission, and she is addicted to the feeling of having a body she can control. Mitchell urges her to stop, as her soul has become more and more entwined with Liam's body every time she uses it; if she persists, she will be trapped there forever, and it is possible she will force Liam's spirit into her place as a passenger in his own body. Eve swears never to borrow Liam's body again.
On an errand for Mitchell, Liam goes to the outermost edge of the wards, where they are weakest. An accidental fall lands him outside of the wards, where members of the Order abduct him. Liam wakes in their headquarters and comes face to face with Eve's killer, DAMIEN ROWLE. Damien reveals that Eve's murder was orchestrated by the Order to complete a Frankenstein-esque experiment involving soul transfer and reanimation. But the plan was botched when Eve's soul entered Liam's body, and now the Order intend to make things right by transferring her to the corpse of Damien's niece.
Partway through the ritual, Mist and Mitchell arrive to save Liam and Eve. Mist holds the Order off with magic while Mitchell frees Liam and flees with him. The battle ends in an explosion which kills Mist and many Order members, and severs Eve from Liam completely. Liam wakes a few days later to find that Eve now inhabits a body which is partly hers, partly that of Damien's niece, and partly made of magic.
Liam returns to Ireland with Eve, now renamed "Misty Woods" in memory of Mist. They return at the moment Eve left, with their parents none the wiser. Liam introduces "Misty" to his parents as an orphan he met in the woods, and they agree to care for her as if she were their own.