In Kingston, New York, a high school girl named Emily Parris seeks revenge against her classmate, Mary Warren, after Mary sabotages her school play audition. While Mary auditions, Emily steals her phone and publicizes all her private texts on her blog. This act of betrayal sparks a fierce retaliation from Mary, who publicly insults Emily by calling her a "blog whore" in front of their classmates. The aftermath of their conflict leads both girls to a counseling session with guidance counselor Gordy Gambhir.
Later that evening, Mary announces her decision to take a "vow of silence" on Facebook and deletes her social media accounts entirely. Fueled by her newfound resolve, she cultivates the idea of forming a "Sisterhood" and successfully recruits classmates Catherine Huang and Lavinia Hall as founding members. Together, they embark on a nighttime adventure into the forest, where they begin their mysterious rituals.
As the new school year begins, rumors of the Sisterhood spread like wildfire, and all the girls wish to join this elusive group. However, the details of their ritual remain a secret due to the vow of silence each member solemnly takes. New girls who are accepted into the Sisterhood receive a paper square as an invitation, chosen by Mary. This inclusion leaves Emily feeling dejected after her attempt to join is thwarted. Determined to uncover the truth, she follows the girls into the woods and hides to witness their ritual. However, she is soon spotted by Mary.
Back in her bedroom, Emily posts a scandalous blog entry claiming to have overheard the Sisterhood engaging in inappropriate activities and even going so far as to assert they harmed her. Her sensationalized account sends shockwaves through the community. In a dramatic confrontation at a Mass, Emily publicly accuses Mary of wrongdoing, leading to even more tension when Mary simply states her intention to distance herself from the Sisterhood.
Despite growing scrutiny from parents, teachers, and media representatives, Mary, Catherine, Lavinia, and the other members remain tight-lipped about their activities. Complications arise when Lavinia's mother, Rose, discovers troubling items in Lavinia's room, including a doll marked with a pentagram, raising further alarms. Tensions escalate as two more girls step forward, alleging that they, too, were abused by members of the Sisterhood, igniting public outrage. Emily and the two girls concoct a plan to pressure Lavinia into confessing the Sisterhood's secrets.
That same night, seeking comfort after a series of personal setbacks, Mary visits her boyfriend, Jeff, whom she cannot reach. Instead, she confides in Gambhir, blissfully unaware that Sue Parris, Emily's mother, is observing their interaction. Following a misinterpretation, Sue sends out a photo of them, leading to Gambhir's dismissal from his position despite insistence that nothing improper occurred between him and Mary.
As frustration mounts, Catherine and Lavinia urge Mary to come clean about the Sisterhood, yet Mary insists she can manage the fallout. When she addresses a press conference, her attempts to alleviate the scandal backfire, especially as Gambhir stands up for the girls. In a significant moment, Mary crafts an infamous invitation paper square for Gambhir, asking him to witness their secret ritual.
At the ceremony, they gather around a fire, sharing their most intimate thoughts and burning pieces of paper inscribed with their secrets. Throughout a montage of Sisterhood meetings, feelings are laid bare: Catherine longs for her ailing mother, Mary wishes to lose her virginity to someone she loves, and Lavinia fears she may never experience her first kiss.
Emily, meanwhile, is invited to discuss the sensitive topic of sexual abuse on a radio show, igniting guilt about her previous actions. She decides against her plan to expose Lavinia but is too late to stop the others, who move forward with their scheme. In a chilling twist, Lavinia is ensnared by her crush, Travis, lured into the woods, where she is subsequently ambushed by Emily's friends. Under duress, Lavinia is forced to declare that the Sisterhood members are witches, and a humiliating video of the encounter is shared online. The bullying intensifies at a Halloween party, culminating in Lavinia's tragic suicide through an overdose of her mother's medication.
During Lavinia's poignant funeral, Mary honors her friend's last wish by kissing Jeff. Overcome with grief, Emily confesses that she fabricated the entire story out of jealousy after being spurned by Mary. She pleads for forgiveness, acknowledging the fallout from her blog lies, yet some still see her as a voice for abuse survivors.
In the aftermath, Catherine embraces self-care by shaving her head and finally visits her ailing mother. Meanwhile, Mary and Jeff mend their relationship. Together, she and Catherine perform a heartfelt dance dedicated to Lavinia, choreographed to her original music, as they move through their town, watched by the community.
As the film draws to a close, Emily stands nearby as Mary extends an olive branch, offering her the Sisterhood paper square once more—a symbol of acceptance. Together, the Sisterhood girls dance through Kingston and into the woods, a poignant reminder of their unity and shared struggles. The narrative concludes with reflections on the Sisterhood’s lasting secrecy, suggesting that their traditions will continue to be passed on, quietly fading into the shadows.
