Note: This entire film is shown through the point-of-view of laptop or phone screens.
The film opens with a photo/video montage showing the life of a Korean-American family known as the Kims - David Kim (John Cho), his wife Pam (Sara Sohn), and their young daughter Margot. Margot took up an interest in piano lessons from a young age. Pam was later diagnosed with lymphoma that went into remission, but she later experienced a relapse. David and Margot stood by her side until she passed away in 2015.
In the present, David chides Margot (now played by Michelle La as a teenager) for not taking out the trash. She says she will do it when she gets back from a study group session. David then talks to his brother Peter Kim (Joseph Lee), who is trying to make a gumbo recipe that Pam used to make. David sends him a picture of the recipe and then takes the time to watch a video of Pam showing a young Margot how to make the gumbo.
That night, as David is sleeping, he misses two calls from Margot.
David messages Margot again for not taking out the garbage. Hours later, he still hasn't heard from her. After work, he tries to contact Margot, but she never answers her phone. He thinks she might be at a piano lesson, so he calls the piano instructor, only to discover that Margot canceled her lessons six months earlier, which means that she has been taking the $100 bills that he has left her for herself. David starts to worry.
David looks up the numbers of Margot's classmates from Pam's old contacts list. He calls the home of Margot's childhood friend Isaac (Connor McRaith), and he speaks to Isaac's mother, who tells David that Isaac and his friends went on a trip to the mountains, so she believes that Margot could be with them. David manages to get in touch with Isaac, who tells him that Margot is not with them and that he had invited her, but she declined.
David calls the police and he speaks to the renowned Detective Rosemary Vick (Debra Messing), who is assigned to the case. She tells David to get in touch with anyone who may have been close to Margot or know where she was last seen. David goes through Margot's laptop and finds that all of her social media accounts are private. He logs into her Facebook profile using Pam's old email address, and he begins to reach out to Margot's classmates. Most of them have alibis from the night that Margot went missing, and others say that Margot was usually quiet and distant. David compiles a chart to keep track of who he has spoken to so that he can show Vick.
Vick later shows David traffic cam footage showing Margot leaving a gas station and heading out of town. She also sends him a fake ID that Margot appears to have used under the name Jen Yeun. This, coupled with the fact that David found Margot's Venmo transactions having taken out $2,500, leads Vick to suggest that Margot ran away.
Growing tired and increasingly afraid, David then comes across the site YouCast where Margot left live-streaming videos. He goes through the saved videos to see that Margot would talk to someone named Hannah, whose username was "fish_n_chips". Hannah also had a sick mom and would relate with Margot. David then notices one of the videos is of Margot at a lake, which matches a picture she posted to her Tumblr account. It's Lake Barbosa, where Margot liked to go to relax. David finds the directions from the spot where Margot was last seen, and he heads out.
David calls Vick in the middle of the night once he gets to the lake. He finds a Pokeball key chain that belonged to Margot, confirming that Margot was there and that she didn't run away. In the morning, authorities fish Margot's car out of the lake. An envelope with $2500 is there, but Margot isn't in the front or the trunk. A bit of blood is spotted on the dashboard.
News of Margot's disappearance spreads, leading to a search party scouring the woods for her. It becomes a trending topic, with some people sharing love and hope for Margot to return, while others mock the situation and accuse David of being involved in her disappearance. One of Margot's schoolmates, Derek Ellis (Reed Buck), who is (for lack of a better term) a douchebag, jokes that Margot is with him and that he's her pimp. David finds out he's at the nearby movie theater after checking in on Facebook, and he goes to confront him, leading to David kicking Derek's ass and breaking his jaw, which patrons record on their phones. Vick tells David to stay off the case because of this.
David looks through the photos of the site of where Margot's car was found. Upon closer inspection, he notices a hockey hoodie in the front seat of the car that he recognizes as belonging to Peter. David looks through Margot's messages between her and Peter, which imply that they have been doing something that Peter knows David would kill him for.
David pays Peter a visit and sets up cameras in his home to try and get a confession out of him. David asks Peter when he last saw Margot, and he appears to dodge the question. David then attacks Peter until he tells the truth, and Peter admits that he and Margot were smoking weed. She had caught him at one point and wanted to try it, so they kept it a secret from David. Peter points out that he was the only one who listened to her when she talked about her mother's passing, since David had avoided talking to her about it. David then gets a phone call from Vick, but he misses it and just hears her voicemail. Vick tells him that they got a suspect, and David breaks down crying as he listens to the voicemail.
The news reports that an ex-convict, Randy Cartoff (Ric Sarabia), has apparently confessed to killing Margot before committing suicide. A video of his confession is shown, in which he tearfully apologizes for what he claims to have done to her. David is sent memorials and condolences for the news.
David receives an email from a memorial service asking for videos and pictures of Margot that he would like to share for her upcoming memorial. After picking all the files that he wants so share, he notices a picture on the service's site that looks familiar. It appears to be Hannah, AKA fish_n_chips. He searches her image on Google and notices that the girl in the picture is some kind of catalog model. David calls the company that Hannah works for, and he speaks to her, but she has no idea who Margot is or what YouCast is.
David tries to call Vick to investigate further since this doesn't add up for him. He instead speaks to a woman from the police station that tells him that Vick is not there, and she lets it slip that Vick volunteered for the case instead of being assigned as he had been led to believe. David does a little more investigating and sees an article of Vick with construction workers, and one of them is Randy. With this information, David brings a group of police officers to Margot's memorial, and Vick is arrested.
A week after Vick's arrest, we see her being interrogated. She confesses that everything she did was because of a mistake that her son Robert (Steven Michael Eich) had made. It turns out that Robert had been in love with Margot for years, and he cat-fished her as fish_n_chips, using Hannah's picture as his own and coming up with this fake persona and backstory for months. Margot had taken out the $2,500 to help with what she believed was his sick mother's hospital bills. When Robert found her smoking weed at the lake, he frightened her and she ran. Robert went after her, and she started hitting him in self-defense, and he reacted by pushing her down the ravine. Robert called Vick panicked, and she told him not to do anything. She made up everything to throw David off, including making a fake ID for Margot, making it look like her body was nowhere to be found, and even setting Randy up to make a false confession and having him killed. Vick then points out that there is a possibility that Margot survived because there was a storm two days after she went missing, so she would not have been totally dehydrated. Robert is also taken into custody following this confession.
A rescue team goes down to the ravine where Margot fell. From a live news chopper, we see the rescuers bringing up a body basket with Margot inside. David goes over to see if Margot is okay...
It's now months later. We see a message exchange, followed by a picture of David with Margot in a wheelchair at the hospital, confirming that she is indeed alive. She is waiting and refreshing to find out if she got into a music conservatory that she had applied to. David messages her telling her that he is proud of her, and that her mom would be too.