After a series of child abductions in the Midwest in the 1980s, an Iowa dairy put pictures of missing children on milk cartons as a public service. It quickly became a national phenomenon for a number of years. Caroline Cooney's 1990 novel is part of the Janie Johnson series of young adult books. Its protagonist, Janie Johnson, spots a picture of her younger self on a milk carton at her school.
Seeing the picture of herself sparks flashbacks for Janie. She remembers the dress from the picture and the vague outlines of relationships with other children and adults. After finding the dress in the attic, she conducts some research and discovers news stories about the abduction of a girl from a New Jersey mall some years before. When she confronts her parents with this information and her suspicions, they tell her that she is actually their granddaughter and that her mother is their older daughter, Hannah. Hannah told them that she was forced into a relationship in a cult she lived in and that the cult would try to take her daughter Janie away if they knew where she was.
Ultimately, Janie's desire to know more leads her to the family of the abducted girl who is, in fact, herself. The family resemblance is too strong to deny. The truth emerges: it was Hannah who abducted Janie. Janie feels relief that the parents who raised her are not kidnappers, and at the book's end, she is set to speak to her biological family with her adoptive parents' blessing.