Four friends band together against an anonymous foe who threatens to reveal their darkest secrets, while unraveling the mystery of the murder of their best friend
Best friends Aria Montgomery, Emily Fields, Hanna Marin, and Spencer Hastings grow apart when their clique leader and 'queen bee' Alison DiLaurentis goes missing. A year later they all begin receiving messages from the mysterious "A", who threatens to reveal their darkest secrets. Aria has just returned from Iceland after a year or so with her family and still feels unable to forgive her father who cheated on her mother, who's still unaware of it; she also discovers that her new English teacher (Ezra Fitz) is the same guy she met and made out with at a bar the day before. Hanna is now the "it" girl of the school, along with former dork Mona Vanderwaal. Both girls indulge in shoplifting; later, Hanna gets in trouble, forcing her mother to sleep with a detective for her release. Spencer has feelings for her sister's fiancé (Wren Kingston). Emily befriends a girl named Maya, who's just moved into Alison's old house. Later, Alison's body is found buried in her old backyard.
Realizing that 'A' couldn't be Alison, the girls began searching for clues. Their initial suspect was Toby, who took the fall for an incident the girls refer to as 'the Jenna thing'. After an attack from her boyfriend, Ben, Emily bonds with Toby. Emily begins to believe Toby may not be Alison's killer, later confessing to him that she is a lesbian. Emily then starts going out with Maya, later developing a relationship with her. Emily later confesses to her parents about Maya and her sexuality, leading to a conflict between her and her mother. Maya was later sent to juvenile detention following the discovery of marijuana in her bag by Emily's mother. Toby is arrested for Alison's murder, but claims that he met Alison the night before she disappeared to thank her for what she did. It is revealed that Toby had a sexual relationship with his stepsister, Jenna. Emily decides to hold a memorial for Alison; however, the memorial was later destroyed, and a picture of her at its remains made her a suspect. Emily subsequently reveals her feelings towards Alison, after Detective Wilden tries to pin Alison's murder on her.
Meanwhile, Aria attempts to tell the truth about her father to her mother, but 'A' tells her mother before she could. This led to her parents' separation and her brother, Mike's, change in behavior.
Meanwhile, Melissa breaks up the engagement with Wren after seeing him kissing Spencer, leading to a conflict between the two sisters.
Hanna attempts to make her boyfriend, Sean, forget the old her, though this backfires.
The girls' next suspect was Noel, after Hanna is hit by a car while spying outside Mona's birthday party. Hanna witnesses Noel writing a message at the back of a car, owned by Ezra. Noel blackmails Ezra about his relationship with Aria, but is later framed by 'A' for cheating on exams. Hanna and Sean later break up, after Hanna dances with Lucas at the homecoming party (as 'A' demanded). Ian returns in town, later reconciling and eloping with Melissa, to the surprise of the girls. Hanna later enlists Caleb's help to work on Emily's phone in order to talk to Maya. She eventually sympathizes with him after discovering his poor home life, and develops a relationship with him. However, she breaks it off after she discovered that he was working for Jenna. Emily comes across Paige, a teammate of hers who began making homophobic remarks to Emily, though she later reveals herself as a lesbian. She and Emily later began dating, although Paige wants to keep things a secret. She and Emily later break up, and Emily dates Samara.
The girls discover that Ian was the guy Alison was seeing the night she disappeared, and suspect he was the one who killed her. After finding more clues, the girls, with Garett's help, set up a plan to blackmail Ian, but this backfires when Ian sends someone to meet them in his place. Spencer is confronted by Ian at the church. He corners her at the bell tower, attempting to kill Spencer. However, a mysterious figure pushes
Ian off the tower, presumably killing him
Ian off the tower, presumably killing him