Inspired by Sara Shepard’s young adult book series, “Pretty Little Liars” centers on the lives of four friends—Spencer, Hanna, Aria, and Emily—whose tight-knit group unravels following the mysterious vanishing of their leader, Alison. A year after her disappearance, the girls start getting mysterious messages from an anonymous sender known as “A,” who threatens to reveal their secrets, including those they believed only Alison was aware of.