Treasure lives with her mother and her abusive stepfather, Terry, whom she hates very much. When Terry whips Treasure with his belt, scarring her forehead, her grandmother Rita puts her foot down and takes Treasure in herself, although she still has nightmares that Terry is coming to get her.
India lives in the upper-class Parkfield estate with her parents and her au pair, miserable Wanda. Her father is a businessman with a terrible drinking problem, and her mother, Moya Upton is a controlling fashion designer who finds India’s weight problem particularly embarrassing. India is unpopular at school and her only friend now goes to boarding school. Her only source of comfort is rereading The Diary of Anne Frank, whom she considers her heroine.
One day, after Wanda fails to collect her from school, India decides to walk home. While passing through the Latimer Estate she meets Treasure, and the two hit it off instantly. India is very taken with Treasure’s charismatic family, and Treasure is impressed by India’s posh lifestyle. Their friendship proves to be the bright spot in both of their lives, as India begins to suspect that her dad is having an affair with Wanda and Treasure receives a phone call from her mum and Terry, telling her that they are coming to take her home.
To protect Treasure, Rita sends her granddaughter and the other kids out of the flat until the coast is clear. However, Treasure is spotted by Terry and has to run away where he can’t find her – to India’s house. India takes advantage of her family’s inattention to hide her friend away in her attic, where she can live “like Anne Frank”. She lends the book to Treasure, who begins to idolize Anne as well.
Meanwhile, word has spread around the Latimer Estate that Treasure has been abducted. Her mother and stepfather broadcast an appeal on the news, and her neighbour’s son, “Mumbly” Michael Watkins, is accused of kidnapping her. Treasure is horrified and wants to go home, knowing that her Nan is worried about her, but India does her best to convince her to stay where she is safe, all the while coping with her own problems, including her father’s alcoholism and the fact that Wanda may be pregnant.
When Nan shows up at India’s school, demanding to know where Treasure is, India has no choice but to reveal their secret. Her mother is horrified, and scolds her, which results in India revealing Wanda’s secret too, out of spite. After Nan promises to keep India’s involvement in the incident quiet, and takes Treasure home, Treasure tells her Nan that she is going to tell everyone the truth about Terry. Her revelation on the news leads to a custody battle between her parents and her grandmother, which Rita eventually wins with the aid of a campaign. Treasure’s mum declares that she doesn’t want Treasure any more because she told the truth about Terry, but they hug each other and Treasure sets out into her new life with Rita.
In the last chapter, India discloses that her parents are getting a divorce after her father narrowly escaped arrest for embezzlement; Wanda was sent back to Australia, allegedly after terminating the pregnancy; and India is in love with her new therapist, Chris, who has encouraged her to pursue a career in psychology. Treasure, meanwhile, is still her best friend: she is now working as a model for India’s mum (Moya Upton, the clothes designer), who takes them on a photoshoot to Amsterdam at the end of the story, complete of course with a visit to Anne Frank’s attic. The sight of the book makes both girls cry; although they cannot read Dutch, India states that it wasn’t necessary as “we both know it by heart”.