No one knows Margo’s real name. Her colleagues and patrons at a small-town public library only know her middle-aged normalcy, congeniality and charm. They have no reason to suspect that she is, in fact, a former nurse with a trail of countless premature deaths in her wake. She has turned a new page, so to speak, and the library is her sanctuary, a place to quell old urges. That is, at least, until Patricia, a recent graduate and failed novelist, joins the library staff. Patricia quickly notices Margo’s subtly sinister edge and watches her carefully. When a patron’s death in the library bathroom gives her a hint of Margo’s mysterious past, Patricia can’t resist digging deeper --- even as this new fixation becomes all-consuming.

Though the two women live just a few doors apart, a chasm lies between them. The actress, a celebrity with a charmed career, shares a gleaming brownstone with her handsome husband and three adorable children, while the recently separated narrator, unhappily childless and stuck in a dead-end job, lives in a run-down, three-story walk-up with her ex-husband’s cat. As her fascination grows, the narrator’s hold on reality begins to slip. Before long, she’s collecting cast-off items from the actress’s stoop and fantasizing about sleeping with the actress’s husband. After a disastrous interaction with the actress at the annual block party, what began as an innocent preoccupation turns into a stunning --- and irrevocable --- unraveling.