Pretty Little Liars is an American teen drama mystery thriller television series developed by I. Marlene King and is loosely based on the novel series of the same name written by Sara Shepard. The series follows the lives of four girls whose clique falls apart after the disappearance of their leader. One year later, the estranged friends are reunited as they begin receiving messages from a mysterious figure named "A" who threatens to expose their deepest secrets. The series features an ensemble cast, headed by Troian Bellisario as Spencer Hastings, Lucy Hale as Aria Montgomery, Ashley Benson as Hanna Marin, Shay Mitchell as Emily Fields, Sasha Pieterse as Alison DiLaurentis and Janel Parrish as Mona Vanderwaal.