Finna's mother is dead. So why is this woman standing on her stoop claiming to be just that? Catriona arrives bearing a gift—a luminous moonstone that holds the secrets to Finna's heritage. What heritage? Before Finna can reply, Catriona is telling her all about a journey the two of them must make to the castle of the Celtic moon goddess, Arianrhod.

Moon goddess?

The story Catriona weaves is preposterous. A parallel reality, known as the 'Otherworld', exists right next to this one. According to the woman's description, this place is stuck somewhere in the past, with no electricity, cars or any other modern conveniences.

"The moonstone will show the way," Catriona intones cryptically.

Newly pregnant Finna is naturally skeptical. Where has this woman been for the past eighteen years? And why would her father lie about her mother's death? All her instincts tell her to stay put during this critical time. But Catriona is uncannily persuasive, convincing her despite all of Finna's misgivings.

What a trip through this alternate reality really means, and the secrets Catriona reveals as they travel, have Finna questioning everything she has ever known. With her life and that of her unborn child in jeopardy, Finna must find a way to survive.