On her fifteenth birthday, Emerson Page finds herself amid chaos in Times Square on New Year's Eve.

Like her deceased mother, a renowned anthropologist, Emerson is a Starlighter-she has a strong internal light that she can call into her hands and use as a tool. Now, trained to use her gift, she is ready to leave her home in New York City and find the treasure her mother died trying to find-a book written by the muses of Greek mythology that holds the secret to limitless human creativity.

Emerson's grandmother, who went missing when her mother was a child, re-emerges with the information about the book's location. The book of the Muses is in the land of the faeries below Dublin, Ireland that's rooted in Celtic mythology and operates nothing like her own world. Emerson discovers she's not the only one who knows about the book's whereabouts. One of her family's closest friends betrays her and sets out to find and take the book for himself. Now they're locked in a race to reach it first.

With her two closest friends and her service dog, Friday, who helps her manage her PTSD caused by the tragic loss of her mother seven years earlier, Emerson falls through levels of the underworld below Dublin. They face physical, mental, and emotional struggles on their journey that push every limit they have. They encounter magical beings from Celtic mythology and convince them to help them reach the faeries in the hopes of persuading them to give Emerson the book.