Marina Nedivar's backstory revealed that she was born after a fire burned down her home (which housed a cult); a fire that left her pregnant mother severely burned. She came out of the womb following her mother's (presumed) death, and she was taken as a ward of the state. As early as the age of seven, Marina dabbled in black mirror witchcraft; causing other children to be fearful of her, accusing her of giving them nightmares. As she grew into adulthood, Marina posted dark images on her Facebook page, doing so under the name Marina Mills.
In the film, Marina attended the same college as main protagonist Laura Woodson, who noticed Marina's Facebook page. After she (along with her friends Isabel and Olivia) noticed that Marina had no Facebook friends, Laura decided to friend her, which immensely pleased Marina. The two chatted face-to-face afterwards, leaving Laura believing that Marina is a nice young girl who just needed companionship. However, as the film progressed, Marina swarmed Laura with various messages, while obsessively stalking her Facebook page. On the night of Laura's birthday, Marina talked to Laura online, showing worry about Laura being upset with her over the overflow of messages. She also attempted to invite herself to her birthday gathering, only for Laura to falsely claim that she was only spending time with her boyfriend, Tyler.
However, after looking at Laura's page and seeing that she was out with all of her friends as well as Tyler, Marina angrily confronted Laura at the cafeteria. In the process, her hood was removed and a large bald spot was revealed; confirming Olivia's statement that Marina rips out her hair. The outburst was the last straw for Laura, who later unfriended Marina. Her friends count back down to zero, Marina angrily closed her laptop, and later on, she committed suicide while hanging herself; recording it online while doing so.
In actuality, Marina's suicide was part of a black mirror ritual, as she burned Laura's photo before hanging herself. The ritual allowed Marina to return as an evil and vengeful spirit, with Laura as her target. Marina's plan was to make Laura as lonely as she was, and she started with uploading her suicide video on Laura's Facebook page. Due to the belief that Laura herself uploaded the video, Laura's friend count begins to decrease, and she becomes a social pariah. Marina later added Gustavo (Isabel's brother) as her Facebook friend, and she sent her swarm of wasps to kill him in an elevator. She later possessed Isabel and blamed Laura for Gustavo's death, which was later posted on Laura's page.
This was Marina's routine throughout the film; killing Laura's friends one by one and uploading them on Laura's page, causing Laura's friend total to vastly decrease. After killing Isabel, Marina went after Olivia, who was hurled out of her window by the wasps. After being found by Tyler and Laura, Olivia is hospitalized in stable condition, but later on, the evil Marina possessed Olivia and rose from her hospital bed. She later attacked a police officer and took his gun, which was used to kill herself in Olivia's body.
The film's climax sees Marina appear to Kobe while he and Laura drove to Marina's former home, believing that to be the site of her suicide, and planned to destroy Marina's laptop (the black mirror Marina used for her ritual). Laura ended up running from Kobe as well, as he attempted to kill Laura out of self-preservation, and following this, the protagonist headed to a factory; recognizing that site as the location of Marina's recorded suicide. Marina kills Kobe (who killed Tyler and pursued Laura), leaving Laura all by her lonesome. She later appeared to Laura and led her to her body and her laptop, which transports Laura into one of Marina's earlier posts, right before Marina attacks Laura.
In the end of the film, Marina ended up possessing Laura, who is now seen dressed and acting similar to Marina. She stares at a group of girls and then at her Facebook page, which has zero friends. The laptop used was Marina's old laptop, and the film ends with the indication that Marina plans on continuing her evil methods, using Laura as her vessel. People even say that they get her friend request
- Marina Mills is similar to Laura Barns, the villainess from 2014's Unfriended. Both were young girls who committed suicide and later returned as evil spirits (successfully) seeking revenge on those who (they believed) wronged them. One difference is that Laura was a bully who ended up bullied herself due to a Facebook post, while Marina was a dark and obsessive psychopath.