A windswept island, dreary and forgotten. A girl with magic in her blood, pushed to the brink of madness.

Cassie Murdoch is haunted by the black-eyed woman that killed her mother. Come find me, she unceasingly whispers to Cassie. Feeling driven to insanity, Cassie is sent away to live with her estranged grandmother. Navigating her new home on Tide Island, Cassie searches for clues to her mother's old life and why she fled her home never to return. With help from her grandmother's secretive errand boy, Henry, Cassie unravels a terrible family secret: a curse that spans generations.

Henry and Cassie follow a series of clues left by her ancestors to find that Tide Island is a dangerous place, the link to a mysterious realm called the Otherworld where the black-eyed woman rules. After learning she is descended from a long line of Sea Witches, Cassie is plunged into a world of spells and secret ceremonies.

Desperate to save herself and her newfound friends from her spiraling control, Cassie searches for a way to break her curse. But in the Otherworld enemies are not as they seem, and Cassie must find whom to trust before those she loves most are lost at the hands of the black-eyed woman.