The film unfolds entirely within the bedroom of Casey Jacobs, and it happens in real time. One evening, Casey is listening to music but notices that her Spotify settings are being tampered with. While skyping with Megan Turner, who is about to go on a date, Casey learns that her ex-boyfriend Nathan has posted a cruel comment about her use of antidepressants on Twitter. Megan confides in their friend Alex Freeman and suggests that he hack Nathan’s account as revenge, but he declines.
After their chat ends, Casey receives an instant message, ostensibly from Alex Freeman, who provides her with a hacked link to Nathan's Twitter account. Casey eagerly posts a tweet about Nathan's erectile dysfunction in revenge. However, the language of the messenger diverges from Alex's usual tone, revealing that the sender is an anonymous hacker who presents themselves as a vigilante who "helps victims of cyberbullying."
The hacker first describes themselves as a "fan," showing Casey's past activity across social media, including her online pseudonym "Chronic Youth," before pointing out that Megan hasn't posted a photo of Casey on her Instagram in a long time. They reveal that they are a vigilante and threaten to intervene to "protect" victims by exposing their secrets. Casey, frightened, excuses herself from the chat, but the hacker triggers a screamer on her computer, taking control of the webcam and addressing Casey in a computer-generated voice.
The hacker reveals nude photographs that Casey took of herself and threatens to post them online if she leaves the room, answers any calls, or if her father—the frequent visitor at the door—enters. Casey tries to covertly message Megan, but the hacker intercepts it by accessing her phone as well, effectively leaving her trapped and isolated. To Casey's horror, the hacker uploads a video that she and Megan filmed revealing their friend Tamara Mathis as gay, provoking a flood of angry texts from Tamara.
The hacker explains that Casey's "Chronic Youth" activity constitutes cyberbullying, including a video she made deriding other haul videos. They reveal that they posted the initial Nathan comment from Nathan's account to bait her. Casey initially dismisses their accusations, insisting that teen behavior is normal, but the hacker rebuts with a stark reminder of consequences. The hacker uses videos and screen-grabs to recount the story of Jennifer Li, a former schoolmate whose singing videos on Vine faced an onslaught of abuse due to Casey's trolling, which triggered a harassment campaign and, ultimately, Jennifer's suicide.
Casey wonders if the hacker is a relative of Jennifer's, tears up with remorse, but the hacker denies this and demands that she admit to being a cyberbully, apologize, and take responsibility for Jennifer's death. Casey does so, but the hacker remains unconvinced and continues uploading both Casey and Megan's nude photos. The hacker announces a grim choice: take an overdose of antidepressants for each image to keep them from being posted, or live with the shame that would follow, along with the loss of her best friend. Defeated, Casey contemplates the pills but notices the hacker's gloating pattern—the taunting "har har har"—matching a known aggressor from Jennifer's story.
Casey pushes back, realizing the hacker has goaded others into suicide. Her Dad arrives with a message from Megan, who has deduced that the online activity isn't truly coming from Casey. Casey, moved by her friend's trust, reaches out again and, despite the threats, vomits up the pills she had taken. The friendship and trust give her the courage to stand up to the attacker: she tells them to post whatever they want, because they can do nothing to control the actions or emotions of her family, friends, or real life. The hacker pleads to reveal their identity, but Casey responds that it doesn't matter—once she stops talking to them, they become nothing. She closes her computer, cutting off the hacker.
Overwhelmed by the hour-long ordeal, Casey opens the door and collapses in the hallway, tearfully calling out to her father, who rushes to her side.
