When a humiliating video of Fresno, California high school student Laura Barns (Heather Sossaman) is uploaded to YouTube, causing her to endure widespread mockery and ridicule, she promptly commits suicide.
A year later, her closest friend, Blaire Lily (Shelley Hennig), views a LiveLeak video of Laura's suicide - where she is seen pointing a gun to her head and shooting herself, despite nearby students begging her to stop. Blaire is then contacted over Skype by her boyfriend Mitch Roussel (Moses Jacob Storm). Both are joined by their classmates Jess Felton (Renee Olstead), Ken Smith (Jacob Wysocki), and Adam Sewell (Will Peltz). During their conversation, however, they notice a random user dubbed "billie227" in their chat, who was not invited in by any of the participants.
After several unsuccessful attempts to get rid of the stranger, the fivesome suspect a sixth classmate–Val Rommel (Courtney Halverson), whom they all despise—is pranking them. Accordingly, they invite Val to their chat. Without warning, Jess's Facebook is updated with photos of Val drinking and smoking at a party. As Val loses her temper, Jess proclaims her innocence while struggling to delete the photos, which suddenly reappear on Adam's account. After repeated threats from Billie, Val calls 911 to report online abuse and then signs off.
The fivesome discover that "billie227" was Laura Barns' Instagram account. Blaire is sent a link that reveals a photo of Laura messaging Val for her to remove the humiliating video and asking for friendship, and Val rejecting her, telling her to kill herself. Multiple Instagram users then slander Val, who is suddenly brought back into the chat, sitting motionless in her bathroom next to an open bottle of bleach. Initially thinking her camera is frozen, they discover that Val is in fact paralysed in some way as her phone is seen vibrating when Blaire attempts to call her. Val then crashes to the floor as the police arrive. At first, the friends believe she just experienced a seizure, but thereafter, based on the codes that the policemen relay to their colleagues, they deduce that Val committed suicide, most likely by being forced to drink the bleach.
Ken finally gets Billie out of their chat using anti-virus software. When Adam tries to call the police, he is warned by a mysterious voice against hanging up. Billie resurfaces with a camera view from across Ken's room. Ken walks over to have a look and freezes in horror. His webcam momentarily disconnects, and then reconnects to show him being thrown about by an unseen force and mangling his hand in a blender, before using the blades to slice his own throat, thus killing himself. Billie then displays the video that caused Laura's eventual suicide; dubbed "Leaky Laura", it shows a drunk Laura face-down on the ground near her house, barely conscious, sprawled in her menstrual blood and excrement.
Billie forces Adam, Blaire, Mitch, and Jess to play Never Have I Ever, stating that the loser of the game will die. All four friends are forced to reveal dark secrets which put them at odds with each other: Jess spread rumors that Blaire had an eating disorder, Blaire got drunk and crashed Jess's mother's car, Mitch kissed Laura soon before her death and reported Adam to the police for selling weed, Jess stole $800 from Adam, and Adam offered to trade Jess' life for his own. Soon after it is also revealed that Blaire had sex with Adam behind Mitch's back on two occasions and Adam forced their classmate Ashley Dane to get an abortion after he date raped her using roofies at a party.
Blaire and Adam suddenly receive secret messages from their printers. Worried that they are still lying behind his back, Mitch furiously demands that Blaire reveal her note to him and threatens to leave if she doesn't. Billie assures Blaire that if Mitch leaves, he will die. In a moment of panic and high stress, Blaire shows her message: "If you reveal this note, Adam will die." Adam suddenly kills himself by shooting himself in the face; as he collapses, his camera reveals his note: "If you reveal this note, Blaire will die."
Billie then asks which of the threesome defaced Laura's grave. When Blaire warns Jess against answering said question, Billie turns off all the lights in her house. Jess hides in her bathroom, while Blaire goes on Chatroulette and gets someone to call the police. Suddenly, the video feed disconnects after Jess is seen screaming and being thrown around the room by an unseen entity. Soon after, Blaire receives a Facebook notification from Jess's account. A video of Jess being fatally choked with her own electric hair-straightener appears, and is then uploaded as an Image macro, with the caption being "looks like she finally stfu".
Billie privately messages Blaire on Facebook, finally wanting her to confess who uploaded the Leaky Laura video. Blaire tries to deny any involvement from her and Mitch, but after Billie continues counting down the seconds she has to admit the truth, she confesses that Mitch did it. Without warning, Mitch grabs his own knife and stabs himself in the head, collapsing and disconnecting his video feed, leaving Blaire all alone.
As Blaire breaks down in tears, Billie - finally unveiled as Laura herself - declares her appreciation for Blaire's honesty. A desperate Blaire tries to remind Laura of their friendship when she was alive, but Laura responds by uploading another video of herself on Blaire's account. It is the same Leaky Laura video, only this time the video continues to show that Blaire was the one recording it, laughing at Laura's misfortune and treating it as a prank. With the truth finally revealed, Blaire's account is soon bombarded with hateful comments from those who denounce her for causing Laura's death. Laura tells Blaire that she wishes she could forgive her before signing off.
Blaire is left alone once again; during the silence, her bedroom door suddenly creaks open. A pair of hands slams her laptop shut before disappearing into the darkness. Blaire calls out to Laura, and without warning, Laura's vengeful spirit lashes out at Blaire, presumably killing her as the screen cuts to black.