At twenty-six, Dahlia Lighthouse is haunted by her upbringing. Raised in a secluded island mansion deep in the woods and kept isolated by her true crime-obsessed parents, she is unable to move beyond the disappearance of her twin brother, Andy, when they were sixteen.

After several years away and following her father's death, Dahlia returns to the house, where the family makes a gruesome discovery: buried in their father's plot is another body--Andy's, his skull split open with an ax.

Dahlia is quick to blame Andy's murder on the serial killer who terrorized the island for decades, while the rest of her family reacts to the revelation in unsettling ways. Her brother, Charlie, pours his energy into creating a family memorial museum, highlighting their research into the lives of famous murder victims; her sister, Tate, forges ahead with her popular dioramas portraying crime scenes; and their mother affects a cheerfully domestic facade, becoming unrecognizable as the woman who performed murder reenactments for her children. As Dahlia grapples with her own grief and horror, she realizes that her eccentric family, and the mansion itself, may hold the answers to what happened to her twin.