Using a laptop he secretly stole from a cybercafé's lost and found, Matias O'Brien communicates with his hearing impaired girlfriend Amaya DeSoto over a Facebook call. Matias shows Amaya the progress he made with his software program 'Papaya,' which translates speech into ASL videos. However, Amaya remains disappointed that Matias spent time on a shortcut instead of directly learning sign language to speak with her properly. The call drops and Matias is unable to get Amaya back.
Matias participates in a webcam game night with his friends Damon Horton, AJ Williams, Lexx Putri, Serena Lange, and Serena's girlfriend Nari Jemisin, all of whom are in different locations. While the six friends remotely play Cards Against Humanity, Matias works on reconnecting with Amaya as well as dismissing repeated messages sent to a Facebook account registered to 'Norah C. IV.'
To everyone's delight, Serena and Nari announce that they got engaged. The news is tempered by the revelation that Serena's mother is still in the hospital suffering from cancer.
With Matias' laptop plagued by persistent technical problems, Damon recommends scanning the hard drive for hidden files. Matias finds a massive archive of videos collected from various surveillance cameras and hacked webcams organized by address.
Amaya finally calls back. She and Matias discuss their communication issues.
Facebook user 'Erica Dunne' sends repeated messages to the Norah C. IV Facebook account until Matias finally responds. The person on the other end reveals that he is not actually Erica Dunne, but the man whose laptop Matias stole. Matias ultimately confesses that he stole the computer and tells the man he will return it to where he found it.
Before the laptop can shut down, Matias receives a message from 'Charon 68' asking to discuss a custom job in a secret online location called 'The River.' Matias looks up 'Charon' to discover he was the mythological ferryman of Hades who escorted the dead across the river Styx. As the other friends finally see what is happening on Matias' screen, Matias admits that he stole the laptop because he wanted a better computer to complete his Papaya program. Matias realizes he stole the computer from 'Charon 4,' as 'Norah C. IV' spells his name backwards. Matias also discovers that Charon 4 has a cryptocurrency account containing ten million dollars in bitcoin.
Seemingly believing he is speaking with Charon 4, Charon 68 commissions a snuff video of a victim dying from trephination. Matias immediately turns off The River.
Matias digs deeper into the hard drive to find a folder labeled 'Contributions.' The six friends are shocked when Matias finds apparent snuff videos of a woman named Agatha chained up in a warehouse as well as a woman named Samantha drowning in an acid bath. Charon 4 intentionally shows part of his face to the camera in the most recent video depicting his kidnapping of Erica Dunne, whose Facebook account was used to contact Matias.
Matias reestablishes contact with Amaya's computer, but Amaya's roommate Kelly greets him instead. The other feeds go dark and Amaya's screen begins glitching as Charon 4 enters the apartment and kidnaps Kelly. Charon 4 tells Matias that he will release Kelly if he gets his laptop back, but will kill her if Matias warns Amaya about what is happening. Charon 4 also threatens to kill Amaya if Matias tells any of the other friends or if anyone disconnects from their webcam game session. Charon 4 explains that Matias must find a way to make Amaya come to him so that Charon 4 can follow her to Matias' location.
When the other feeds come back, Matias tries explaining to his friends that he has been working on an immersive horror game and everything they witnessed was part of an elaborate prank. Although they have numerous questions, Matias eventually convinces everyone that he fabricated everything. After much imploring, Matias also convinces Amaya to come to his place just as Charon 4 instructed.
While game night resumes for everyone else, Matias verifies that 17-year-old Erica Dunne is a legitimate missing person. Acting on an idea from Damon, Matias drains the ten million dollars from Charon 4's bitcoin account. Matias tells Charon 4 that he will only return the money if Erica Dunne returns home safe. Charon 4 warns that The Circle of Charons will never allow what Matias is asking.
Suspecting that Charon 4's account was hacked, The Circle forcibly pulls Matias into The River and demands that he recite their code to prove his identity. Fearing that they will both be killed if The Circle discovers his laptop was stolen, Charon 4 shows Matias his face and tells him what to type.
The Circle demands to know why the bitcoin account was suddenly drained. Posing as Charon 4, Matias claims he was merely switching to another form of cryptocurrency. More Charons join the anonymous group call as Matias is questioned.
Having suspected something was off about the videos Matias claimed weren't real, Nari confirms that Erica Dunne really is missing. Meanwhile, the other friends notice that Lexx is away from her computer. A YouTube video suddenly plays on everyone's screens showing a Charon pushing Lexx off a building's roof.
Because Charon 4 hacked Amaya's phone to monitor Matias, Matias directs Amaya to take the subway so that her signal will be lost. During the service interruption, Matias frantically explains that everything the friends have witnessed is in fact real. Matias further explains that to keep everyone safe, they must pretend as though nothing is wrong when Amaya's signal comes back.
Once the Cards Against Humanity ruse resumes, Damon gains admin access to Matias' laptop to hack the computer from his London location. Pretending to leave for a different reason, Nari makes her way to the subway to meet a friend on the LAPD.
Charon 1 hacks AJ's YouTube channel. Charon 1 splices a series of voice clips into a threat that he phones in to 911. Additionally hacking AJ's computer speakers to simulate a shotgun pump, Charon 1 tricks responding officers into breaking down AJ's door and violently shooting him to death.
Charon 4 realizes that The Circle knows he lost his laptop. Charon 4 claims that everyone will be killed, including him.
A Charon presents Serena with video feeds of Nari waiting for a subway train and Serena's mother in her hospital bed. The Charon demands that Serena choose which woman should die, but Serena refuses. Serena's mother dies when her life support system is remotely deactivated. A Charon pushes Nari in front of a subway train, although Nari manages to pull the Charon with her. Serena is then murdered in her home.
Damon threatens to expose The Circle's cabal as he continues downloading the files from Matias' laptop for evidence. The Circle has Damon watch as they edit the Erica Dunne kidnapping video to put Matias' face over Charon 4's. Damon realizes that The Circle planned for the laptop to be found all along so that they could frame the friends for the Charon crimes. Charons plant Erica Dunne's unconscious body in Matias' apartment after Matias leaves to look for Amaya.
A Charon infiltrates Damon's London flat and hangs him. The Charon then forges a suicide note confessing that the friends committed the murders.
Matias then gets a call from Amaya outside an unfamiliar location asking where he is, saying he changed the meeting spot. Matias realizes the Circle has hacked his messages to make Amaya go to the factory from an earlier video. Before he can warn her to run away, the Circle hack his phone and launch the Papaya app, making it tell her to go inside. The call is forcefully hung up, and it cuts to a camera feed inside the factory. Matias watches Amaya get attacked by a Charon, presumably to torture or kill her for another video. Broken, Matias asks the Circle why they did all of this, and they repeat a clip of him earlier saying "After all, it is game night", revealing that this is simply entertainment to the Circle members.
The Circle members launch a poll to decide Matias's fate. The camera switches as Erica Dunne wakes up in Matias's apartment and goes to the computer, begging for help. She pulls a cork out of her forehead to reveal the hole in her skull from trephination and freaks out before the video cuts off. A camera feed of Matias in the road is added as the poll finishes with the Circle deciding his death. As he looks at his phone, he is run over by a Charon in a large van. With their job done and their crimes pinned on Matias and his friends, the members of the Circle at each death scene go to their cameras and take off their masks, smiling, including Charon IV (whose apparent anger and paranoia demonstrated to Matias earlier in the film were clearly just an act to keep the game going), as it is revealed that the audience's point of view was actually from a Charon's computer.
Alternate ending (a.k.a Buried Alive) In the second ending, Matias texts Amaya to meet him in a familiar spot; where he filmed a video of them hiking and sharing a kiss. He arrives at the site and finds a hole dug in the ground with an open casket, before a Charon knocks him out.
Later Amaya arrives and the hole has been filled. She calls Matias and his ringing phone wakes him up. He realizes that he has been buried alive and tries to text Amaya about his predicament but the Circle changes all of his messages to: "I wish I could sign better." When he attempts to video chat, the Circle pixelates his mouth so Amaya cannot read his lips. Amaya gets mad and hangs up, leaving Matias to die as he runs out of air.