A year before the beginning of the movie, high school student Laura Barns committed suicide following unrelenting harassment that she suffered when a video of her having soiled herself after getting black-out drunk at a party leaked onto the internet.
Blaire Lily, who was Laura's friend when she was alive, is watching a video of the suicide on LiveLeak when she is contacted via Skype by her boyfriend, Mitch Roussel. As she does a striptease for him and promises to give him her virginity on prom night, they are joined in the Skype call by their friends: Jess Felton, Ken Smith, and Adam Sewell. The light-hearted fun conversation between them soon ends when they notice someone else had joined their call, using a default avatar and the screenname "billie227". All attempts to expel "billie" from the call prove futile. At the same time, Blaire is contacted via Facebook private messages by Laura's account. Attempts to have the account reported or memorialized also fail.
The group begin to suspect that they are being trolled by a classmate, Val Rommel. After inviting her into the call in an attempt to clear the air, photos of Val drinking and smoking at a party are uploaded to Facebook via Jess's account. Jess claims to be innocent and tries to delete them, only for them to be re-uploaded via Adam's account. Eventually, "billie" talks to the group and claims that if any of them disconnects from the Skype call, they will die. Val angrily calls the police and hangs up from the call.
Afterwards, the group do research into "billie" and discover that the Skype screenname is the same as Laura's Instagram account. Blair also receives a link to an off-screen picture of an IM conversation between Laura and Val, with Laura's attempts to extend an olive branch to Val being met with demands for her to commit suicide. The comments section of the picture are filled with hatred towards Val.
Val suddenly rejoins the call, but is seen sitting motionless in the utility room of her house, a mirror behind her broken and an open bottle of bleach next to her. When Blaire calls her cell phone, Val continues to sit there as her phone vibrates and moves. Eventually, she and her webcam fall to the ground with a clatter. Police arrive moments later, calling in EMS for what they deem to be a suicide.
Ken tries to expel "billie" from the call by emailing the other members of the Skype group a self-produced Trojan virus removal program. It seems to work, with "billie" disappearing from the call. Their relief is short-lived, however: "billie" rejoins the call, this time with a camera feed from Ken's house. When Ken goes to investigate the source of the feed, what he finds horrifies him into silence. He disconnects moments later, and when he reconnects, he is seen being accosted by an invisible force before slashing his hand and his throat on the blades of his blender.
"Billie" then commences a game of "Never Have I Ever". The rules are simple: each player has to hold up their hands, and for every ill act that "billie" states in the Skype's chat, the guilty party has to lower a finger. The loser of the game will die, according to "billie".
Dark secrets are revealed about each friend over the course of the game, from Blaire wrecking Jess's car while driving drunk to Mitch ratting Adam out to the police for selling pot. Adam, drunk and belligerent, takes advantage of the "game" to reveal that he and Blaire had cheated on Mitch, having had sex twice, infuriating Mitch to no end.
Blaire and Adam then receive messages via their printer, but refuse to show them to the others. Mitch threatens to break up with Blaire and leave the Skype call, in spite of threats from "billie" that he will die if he leaves. Reluctantly, Blaire revealed the message she was sent: "If you reveal this note, Adam will die." True to the note's threat, Adam shoots himself. His webcam falls, revealing that he had received a similar message threatening Blaire if he revealed it.
Next, "billie" wants to know who desecrated Laura's grave. Blaire warns Jess not to answer, and "billie" retaliates by shutting off the lights in everyone's house. Desperate, Blaire turns to Chatroulette in the hopes of finding someone who can call their local police and have a patrol car sent to Jess to ensure her safety. It takes her plenty of tries, but she eventually finds a person who calls the police for her. After succeeding, however, Jess disconnects. She reconnects moments later, also accosted by unseen forces before choking to death on a hot hair straightener.
Afterwards, "billie" IMs Blaire via Facebook demanding to know who uploaded the video that lead to her suicide. Blaire tries to profess her and Mitch's innocence, but via continued threats, finally admits that Mitch uploaded it. As she does, Mitch suddenly stabs himself through the eye with a kitchen knife.
"Billie" finally reveals that she is Laura, and thanks Blaire for her honesty, but asks for her to come clean. Blaire tries to appeal to their friendship, but a video is suddenly uploaded to Laura's memorial page on Facebook via Blaire's account. The video is of her, black-out drunk and having soiled herself, but with extra footage at the end that reveals that Blaire was the one who originally filmed her.
As the video's comment section is filled with hate for Blaire and demands for her to kill herself, Laura sorrowfully comments that she wishes she could forgive Blaire, then leaves the Skype call.
Blaire sits shell-shocked, then after a few moments, ghostly hands suddenly slam her laptop shut. When she looks up, she is attacked by Laura's vengeful spirit.