Ireland 1841. As her parents sleep, an infant girl is snatched away by the hands of the creatures from the Deep Dens in the western ocean-the fomori. Years later, she is mysteriously saved from them, and surfaces near the shores of Kilkee. New to the 19th century world where she once belonged, she uncovers that a changeling has taken her place and stolen her life.

To get justice, Maggie must find and face the changeling before the King of the Fomorians locates her. As the serpentine blood runs through her veins, Maggie fights her animalistic tendencies as it begins to change her body back into its monstrous form. She finds the changeling in Dublin, but a powerful banker with ties to the Dublin underbelly, practices sorcery to use the changeling for his own transcendence toward godhood, making the hunt far more dangerous. As the fomori manipulate the circumstances to draw her into a trap, she must escape them or be taken back into the hellish depths of the sea and suffer its terrors for the rest of her life.