Upon receiving a mysterious letter that her mother's grave has been vandalized, Marie travels to the desolate island town where she's buried. Just as she arrives, the island closes for the season, leaving Marie trapped in a nightmare.

In a modern America where witches are real and witchcraft is illegal, a sheltered teenager must face her own demons and prejudices as she helps two young witches avoid law enforcement and cross the southern border to asylum in Mexico.

Finding herself the victim of a violent monster attack, Jane launches a vigilante campaign to hunt the beast that tried to kill her. Jane's efforts intensify, but her troubling history of drug use and mental illness bubbles to the surface causing her family, community, and authorities to question the authenticity of her account. Suddenly alone in her fight, Jane starts to doubt her own memory of the attack...to doubt if Monster exists at all.