A dual-timeline psychological thriller set primarily in an isolated New Hampshire cabin. In the present ("Now"), Casey Carter has recently lost her father and been fired from her job, and moves into a remote, storm-battered cabin to reevaluate her life. During a violent storm, she discovers a young, blood-covered girl hiding in her shed, wielding a knife and refusing to explain who she is or where she came from. As the storm cuts off phone and power, Casey is forced into uneasy cohabitation with the girl (who calls herself Eleanor, later revealed as "Nell"), even as she suspects a dangerous intruder may be loose in the woods. In alternating chapters ("Before"), the novel follows a girl named Ella, an abused, neglected middle-schooler bullied at school and living with a hoarder mother — chapters that gradually reveal Ella's traumatic path toward the woman Casey has become. McFadden builds the novel around a layered set of identity reveals: the relationship between Ella, Casey, and Eleanor is deliberately obscured through misdirection, and the "intruder" of the title turns out to be less a straightforward threat than a mirror of Casey's own unresolved past. The book leans into McFadden's signature structure — short chapters, escalating reveals, a claustrophobic setting — and deals with heavier material (childhood abuse, violence, survival) than some of her lighter thrillers, with mixed reader reception: some found the payoff satisfying, others felt the misdirection grew confusing or repetitive.