1978: at her renowned treatment center in picturesque Vermont, the brilliant psychiatrist, Dr. Helen Hildreth, is acclaimed for her compassionate work with the mentally ill. But when she's home with her cherished grandchildren, Vi and Eric, she’s just Gran—teaching them how to take care of their pets, preparing them home-cooked meals, providing them with care and attention and love.

Then one day Gran brings home a child to stay with the family. Iris—silent, hollow-eyed, skittish, and feral—does not behave like a normal girl.

Still, Violet is thrilled to have a new playmate. She and Eric invite Iris to join their Monster Club, where they catalogue all kinds of monsters and dream up ways to defeat them. Before long, Iris begins to come out of her shell. She and Vi and Eric do everything together: ride their bicycles, go to the drive-in, meet at their clubhouse in secret to hunt monsters. Because, as Vi explains, monsters are everywhere.

2019: Lizzy Shelley, the host of the popular podcast Monsters Among Us, is traveling to Vermont, where a young girl has been abducted, and a monster sighting has the town in an uproar. She’s determined to hunt it down, because Lizzy knows better than anyone that monsters are real—and one of them is her very own sister.