In the upper class suburb of Rosewood, Pa., a mystery haunts the lives of four high school girls — Spencer, Emily, Aria and Hanna. It began the summer before eighth grade. The fifth member of their clique, Alison, disappeared at a slumber party. She was never found. Since that time, the girls have gone their separate ways, each mourning Alison, but also relieved at their friend's disappearance because she alone knew all their secrets. Alison's family moved away, and eventually the police stopped actively pursuing clues in the case.
Emily's mother insists she bring a basket of welcome goodies to the family moving into Alison's old house. Emily is surprised to see items from Alison's bedroom stacked on the curb but figures her friend's parents must have been reluctant to throw away their daughter's personal items. A teenage girl approaches and introduces herself as Maya. She, too, will be attending Rosewood Day School, an exclusive private school, the same as Emily and her former friends.
Maya invites Emily up to her room, which is Alison's old bedroom. She tells Emily about her former life in California and the boyfriend she left behind. Emily doesn't tell Maya about Alison's disappearance but does say that she grew apart from her middle school friends. Maya offers her a hit from her marijuana joint. As Emily chokes on the smoke, she notices a long scar down Maya's wrist. Before she can ask about it, they are interrupted by a loud noise outside. Maya explains that the noise is coming from the workers doing renovations in the yard. Emily leaves without giving the family the welcome basket.
Aria and her family return from two years in Iceland, where her father helped with a TV documentary. Aria had reveled in the freedom that life in Iceland gave her and is not looking forward to life back in the States. She had never felt comfortable with who she was until she'd moved away. In Iceland she grew up quickly, traveled around Europe and fell in love several times. Her mother asks Aria to drive her brother to lacrosse practice and wait for him. Aria recognizes several of the boys she used to know who never paid her any attention. They notice her now, but she finds their flirting annoying. She remembers there's a bar nearby and decides to see if she can get a drink while she waits for her brother. The bartender looks at her questioningly but doesn't ask for an ID before serving her a beer. Soon she is chatting with a young man at the bar. He offers to buy her a scotch when she voices her disgust with the taste of American beers. The two talk about Iceland and Europe with Aria pretending to be in college, rather than high school. The man, Ezra, explains that he just graduated and will be teaching English. Aria assumes he will be teaching at the same local college as her father. The two make out in one of the bar's bathrooms.
Hanna, always the ugly duckling among her old group of friends, has blossomed into a swan. After getting cut from cheerleading tryouts before her freshman year, she and another girl, Mona, decided to work together to lose weight so they could try out again the following year. They succeeded in losing weight but never bothered to try out for the squad again. Hanna never admitted to her new friend that she lost weight by binging on food and then throwing it up. In fact, the only person who knew she'd ever done it in the past had been Alison. One day at the mall, Hanna and Mona decide to window shop at Tiffany's. But the girls do more than look; they shoplift.
Spencer has been living in the shadow of her older sister, Melissa, for as long as she can remember. She and her parents have driven to Philadelphia to take Melissa and her new boyfriend out to a celebration dinner. Melissa has graduated from college early and has been accepted to a prestigious business school. Their parents are renovating their city townhome to give her as a congratulatory gift. They offer to let Melissa and her boyfriend, Wren, move into their converted barn while they wait for the work to be done. Spencer is upset because she'd been promised the barn house as her bedroom. She pretends indifference but excuses herself from the table to go outside and have a cigarette. She reminisces about the only time she'd gotten revenge on Melissa by making out with her sister's boyfriend. Only Alison knew about what she'd done. Wren finds her outside and the two talk. Spencer finds herself attracted to him and thinks he may be flirting with her as well. The two return to the restaurant before they act on their feelings.
Aria hates the politics and angst of her first day back at school. She's surprised when she sees Hanna again, impressed at her old friend's new beauty, but they do not reconnect their friendship. Aria daydreams about making out with Ezra in the bar and how she sent him a haiku the previous night. She is shocked when she discovers he is her new English teacher. He swears under his breath when he sees her. Aria receives a cryptic message on her cell phone from someone named "A," alluding to her tryst with Ezra.
Emily, who has been showing her new friend around the school, decides she should walk Maya home as well. Maya has been asked to play guitar in another student's band. She tries to convince Emily to join as well, but Emily has swim practice every day after school. She would be considered a failure to her family if she didn't receive a swimming scholarship as her older siblings have done. In Maya's back yard, the two friends hug goodbye. Maya then leans forward to kiss Emily's cheeks, joking that she's acting like the French. Emily kisses her in the same exaggerated fashion. Maya then kisses Emily again, only this time, they almost touch lips before Maya runs into her house. Emily finds a note in her swim locker from someone named "A" hinting about the kiss.
Spencer returns home sore after the first day of field hockey practice and already burned out from school. She opts to sit in the family's outdoor hot tub to relax. She no sooner steps in before Wren appears wearing only his boxer shorts. He explains that Melissa dropped him off while she ran an errand. He offers to go inside but Spencer allows him in the hot tub. When she complains of a sore shoulder, he offers her a massage. She agrees. When she finds herself excited by his touch and realizes her sister will be home soon, she flees the tub. In her room, she receives a text from "A" warning her against coveting and intimating that Spencer has coveted her sister's boyfriend in the past, a secret only Alison knew.
Hanna is frustrated that Sean, her boyfriend, has taken a virginity pledge because she feels the best way to express their love is through sex. She recalls the feelings of worthlessness she felt after her father left her mother. His new girlfriend has a beautiful daughter Hanna's age. Alison had come with Hanna on the disastrous weekend she first met them. Hanna's father had humiliated her in front of the other girls when he commented on her weight. Her thoughts are interrupted by a police officer at the door. Hanna recognizes him as Darren Wilden, a former Rosewood student with a reputation as a ladies' man. Darren takes Hanna to the police station as the security cameras at Tiffany's caught her shoplifting. Her mother convinces Officer Wilden that the theft was done out of spite because of an argument she'd had with Hanna. Hanna receives a text from "A" warning her that prison food will make her fat and reminding her of an old joke the clique of friends would say.
Aria’s father senses her tension. He believes it’s because being back in America reminds her of the secret they share about his infidelity. She and Alison had seen her father making out in his car with one of his students. Aria never spoke to anyone of what she’d seen. She’s convinced that’s why her father took the sabbatical to Iceland. Since then, her parents’ marriage seemed to have been revived. At school, she tries to convince Ezra to continue their relationship in secret, but he refuses.
Maya convinces Emily to skip swim practice to go on an adventure with her. The two drive out to a stream in the forest and go swimming — Maya in her underwear, Emily in her tank top and undies. Maya admits that she’s thinking of breaking up with her boyfriend in California because he’s being over possessive, and she’s also not sure how she feels about boys in general. She intimates that she’s more comfortable around girls. Before Emily can respond, they are interrupted by a group of boys from the school. Emily hurries to put on her clothes before they see her. Maya, not embarrassed at all about her lack of clothing, takes her time climbing back up the hill.
Hanna and her friend Mona have lunch at the country club. They see Darren Wilden playing golf. Hanna confesses her trip to the police station after Mona teases that she is going to ask Darren to a big party. Hanna binges on a large order of sweet potato fries on the way back to her house because she's afraid others may find out about her shoplifting. Once home, she feels the familiar urge to purge herself.
Spencer can’t wait to tell her family the news about her near perfect PSAT scores, but her sister, Melissa, monopolizes the conversation. Spencer’s chance comes around the dinner table when her father suggests they play a family game where everyone tells something special they did during the day. Spencer is almost forgotten, but when she shows her scores to her parents, they offer to do something special for her to celebrate. Spencer gets up the courage to ask to move into the barn house instead of Melissa and Wren. Her parents agree, telling Melissa and Wren they can share one of the house bedrooms instead.
Aria tries to impress Ezra with her cultural experiences during an oral report, but he cuts her short. She receives another text from "A" teasing that maybe Ezra likes to fool around with a lot of students, just like Aria’s father. The following day Aria flirts with Noel, her partner on an English assignment. She can tell her antics make Ezra jealous. She agrees to attend Noel’s party that night.
Emily has been struggling with her feelings for her boyfriend and those for Maya. Her mother has noticed the change in her behavior and her skipping swim practice. When she pushes Emily to explain what’s going on, Emily confesses that she doesn’t want to swim anymore. Her mother doesn’t think Maya is a good influence on Emily because she and her family are black. Although her parents want to ground her for lying, they agree to let Emily go to Noel’s party with her boyfriend because they hope he will convince her to stay on the swim team. That night, she runs into Aria who asks her about Ali’s disappearance. Before the two can really discuss things, Maya appears and Aria slinks away. Emily abandons her boyfriend to go off and drink with Maya. Later, the two slip into a photo booth Noel has rented for the evening. Emily shares that she once kissed Alison and wrote her a love letter, but Alison ignored her advances. Maya shares that she used to cut herself to relieve the pain she felt. Emily and Maya share several passionate kisses before Emily’s boyfriend finds them. He throws a cup of beer at them and runs away.
Aria drove her little brother to the party but is now bored. Ezra surprises her by calling her cell phone and asking her over to his house. When she arrives they almost immediately begin making out on his bed. Although they get as far as taking off each other’s clothes, they stop short of having sex. Naked, they lie on his bed and talk. When Ezra steps into the bathroom, Aria’s cell phone chimes several times to indicate incoming messages. They are all from "A" and tease her about getting "extra credit" from her teacher. They also wonder what Aria’s mother would do if she found out about her father’s past affair.
Spencer gladly stayed home from the party so she could study in her new barn bedroom. Wren comes over while Melissa is sleeping, supposedly so he can get a tour of the barn. The two begin kissing in Spencer’s walk-in closet, eventually lying on the floor and taking off each other’s shirts. Wren stops for a moment to use the bathroom. While he’s gone, Spencer sees that she’s gotten a lot of instant messages on her computer. Another one pops up in large font. They’re all from "A" and tell her she shouldn’t be kissing her sister’s boyfriend. As Wren comes out of the bathroom, Emily sees Melissa peering through the window. Melissa stomps out her cigarette, and then orders Wren to follow her back to the house. In the morning, her parents insist Spencer move back into the house while Wren is asked to leave. Once Melissa’s townhome is renovated they will turn the barn space into a pottery studio for their mother.
Hanna tries to seduce Sean into having sex with her at Noel’s party, but he refuses. When he strips down to his boxers in order to go swimming, Hanna steals his pants. The keys to his father’s BMW slip out. Hanna decides to punish him by taking the car for a joyride with her friend Mona. The two get drunk, stop at a convenience store for snacks and then crash the car into a tree. Neither girl is hurt. They abandon the car and walk home. In the morning, Officer Wilden again takes Hanna to the police station for stealing and wrecking the BMW. Her mother arrives and promises to take care of everything. Before Hanna can be booked for her crimes, Officer Wilden is called away on a more urgent matter.
Emily decides to go for a bike ride, hoping it will help her sort out her feelings for Maya. In the spokes of her bike is the love note she’d written to Alison years ago. On the back, "A" has written another note. Desperate for answers, Emily disobeys her parents’ grounding and bikes to Aria’s house. Neither girl has the nerve to admit to receiving the strange texts, as they are afraid they’ll have to reveal the secrets they’ve kept hidden for years. Emily bikes over to Maya’s house only to find it surrounded by police cars. Workers renovating the back yard for a tennis court have found Alison's body.
Aria flees to Ezra’s house looking for comfort after she hears the news. He is furious with her and insists she leave. When she presses him for a reason, he tells her that she’d left her cell phone at his house and it wouldn’t stop beeping until he opened the messages. He read the texts from "A" and believes Aria has been playing games with him.
Spencer has no one to talk to about Alison’s death as her family won’t talk to her after her tryst with Wren. Wren sneaks back to the house to see her when the others go out. He wants to continue seeing her and promises to call her soon.
Hanna is home at night watching the news about Alison’s death. She contemplates how her friendship with Alison and the others deteriorated after something she alludes to as "The Jenna Thing." The other girls have thought about this incident as well, with none of them giving explicit details other than someone named Jenna being hurt by a practical joke they had played on her. Hanna’s thoughts are interrupted by the sudden appearance of her mom and Officer Wilden. The two are kissing passionately, and he is trying to undress her. Although no words are spoken, Hanna knows her mother is doing this so that the police will drop the charges against her.
The girls all meet up at Alison’s funeral where they sit up front along with Alison’s parents. The coldness between them begins to melt as they reminisce about the past. The girls become nervous when they see Jenna at the back of the church with a seeing-eye dog, but she disappears before they can reach her. They worry they might still get in trouble for what they did to Jenna. All of them receive a text message at the same time. It’s from "A" and warns that she is still around and knows everything.