The story introduces an exclusive group of friends: Alison DiLaurentis, the perfect yet manipulative queen bee, Aria Montgomery, an independent girl who is considered to be the weirdo in Rosewood, Emily Fields, a swimmer who holds secret feelings for Alison, Hanna Marin, who strives to be thin and popular like Alison, and Spencer Hastings, an ambitious girl who is brave enough to stand up against Alison's manipulative ways. Alison mysteriously disappears during a sleepover with the girls before their 8th grade school year begins.

The story then jumps 3 years after, where the girls are now separated. Aria returns to Rosewood from a 2 year trip to Iceland with her family and comes back much more sophisticated than before she left. She meets and kisses a guy in a local pub who is revealed to be her English teacher, Ezra Fitz. Emily befriends the new girl in town, Maya, and soon develops romantic feelings for her. Hanna is the new popular girl in town alongside the former nerd, Mona. However, she will sometimes shoplift and throw up her meals to stay thin and popular, ruling as queen bee of Rosewood Day. Spencer continues to struggle against the rivalry with her and Melissa, her perfect older sister. Problems occur when she begins to feel attracted to Melissa's new boyfriend, Wren Kim, when he is introduced to the family.

Throughout the story, the liars get messages threatening to reveal their secrets of the present and past, including a terrifying incident the girls refer to as "The Jenna Thing". They automatically believe it is their missing friend, Alison, because she is the only one each of them confided to with their darkest secrets. However, they are shocked when the police find her corpse hidden in the backyard of her former house. The book ends with the liars getting cussed by receiving a text saying, "I'm still here, bitches. And I know everything. --A"