Teenage siblings David and Jen are forced to spend the summer with their elderly aunts in the small town of Aylesbury, Massachusetts, when their father is sent away on business. But their quiet stay ends when David has a mysterious vision in the attic. Looking for answers, they turn to a Ouija board, which leads to the uncovering of a murder and the answer to a decades-old disappearance. They befriend Helen, a studious girl who works in the library, and there they discover De Disciplina Nefasta, The Book of Forbidden Knowledge, an ancient text of spells. With this book, they unintentionally awaken the spirit of a long-dead ancestor, whose spirit has been trapped in a tree for decades, and whose selfish needs threaten to snuff out their lives.