Lexy's mother has recently fallen ill with a motor-neuron disease, and now needs to be constantly monitored and uses a screen to communicate. Lexy's younger brother, Baker, is somewhat of a genius, and builds robots in his spare time. Her father has become the object of attention from the single women in the neighborhood. Lexy, a pretty and popular fifteen-year-old, feels unmoored and different from the kids around her after her mother's illness began.

One day, a handsome older stranger named Dean approaches Lexy at the park. Though he initially seems to be a harmless homeless man, he begins to pop up around her house, and frightens Lexy even though he and his stories intrigue her. Eventually he comes into their yard and talks about how he once lived in their house. Unnerved, Lexy tells her father, and the next week, the family is visited by the former owner of the house, who tells them that Dean was the boyfriend of her daughter. He was never a problem, but was possibly manic-depressive, and had issues at home. When she tells Lexy that her daughter's name is Ally, which Dean had mentioned being a better nickname for Lexy than her own, Lexy realizes that Dean may have fixated on her because she reminds him of his ex-girlfriend.

At the end of the summer, Lexy's mother goes to the hospital and stays in the ICU for a week. That week, Dean leaves groceries on their front porch. Lexy immediately knows that he did it and goes to see him at the park. She confides in him about her feelings in regards her mother's situation but is interrupted when her father arrives in the car. Lexy is grounded, and her father gets a restraining order against Dean.

Her mother comes home from the hospital, and when Lexy goes to check on her one day, she tells Lexy that someone is there, in the room. Dean steps out, and Lexy screams and hits the panic button, which calls the police. He is gone before the cops arrive but is found a few blocks away. He is admitted to a hospital and given medication. The family increases their security around the house. It is set off one night by an opossum, and Lexy remembers her conversations with Dean.