This novel is about a clique of girls who's best best friend goes missing one night at their sleepover. Three years later the girls, who have grown apart, begin to recieve notes from someone named A about all of their deepest, darkest secrets. The girls are positive it is their old best friend Alison, but then her body is discovered in her old backyard. Eventually the notes begin to rule the girls' lives...

This story starts out with 5 best friends, until one night, the leader Ali, disappears. The book then fast-forwards 3 years to the separate lives of the 4 former best friends: Aria, Spencer, Hanna and Emily. They individually try to cope with the most likely conclusion that their former friend is dead. But, when they all receive text messages, emails, IMs etc.. from a character called "A", the girls just know their friend is back. They come together to find her and see her again, and they are confident in their theory that she is back. Until a body is found in Ali's old backyard, that is said to have been there for 3 years... Hmmmmm.....

When one of their tightly-knit group mysteriously disappears, four high school girls find their friendship difficult to maintain when they begin receiving taunting messages from someone who seems to know everything about their past and present secrets.