Mommy blogger Stephanie Smothers (Anna Kendrick), a single mother in Connecticut, makes a video for her mom vlog, "Hi Moms!" Before she begins with her recipe, she updates viewers on an ongoing case involving Stephanie's best friend, Emily Nelson (Blake Lively), who has been missing for the last five days. Stephanie is overcome with emotion before composing herself to give new viewers a recap on what has happened.Stephanie admits she has not known Emily that long, and met her at their sons' kindergarten recently. Stephanie is always volunteering and contributing to Miles' (Joshua Satine) class, to the point where some other parents like Darren (Andrew Rannells), Sona (Aparna Nancherla), and Stacy (Kelly McCormack) think Stephanie is making them look bad. The other parents discuss their jealousy over Emily, who works in New York City for a major fashion brand as a public relations manager. After school, Miles wants to have a playdate with his friend Nicky (Ian Ho), Emily's son. Stephanie first meets the sophisticated Emily when she goes to pick Nicky up, and, at the boys' pestering, Emily agrees to the playdate and invites Stephanie to come over for drinks.Stephanie arrives at Emily's stylish home. She is stunned to see a graphic nude painting of Emily hanging on a wall. Emily is blunt and rather crass, even telling Stephanie to stop apologizing. Stephanie explains that she is a widow after her husband and brother were killed in a car accident. Emily's husband, English professor and one-time author Sean Townsend (Henry Golding), arrives home and meets Stephanie while he and Emily kiss passionately. Stephanie says she has read Sean's book and enjoyed it. However, when Sean isn't there, Emily notes that Sean hasn't written a thing since the first book 10 years ago, and that they are close to bankruptcy.While taking the boys to the park, Stephanie snaps a photo of Emily for the school's yearbook. Emily firmly orders Stephanie to delete the photo, and does not explain why. Stephanie meekly obliges. At Emily's house, the two have martinis and trade secrets. Emily tells Stephanie of how she and her husband got into a threesome with Sean's teaching assistant a few months earlier. Stephanie then tells a secret of how, after her father passed away when she was a high school senior, she met a son he had from another relationship named Chris (Dustin Milligan). In her grief, Stephanie turned to Chris, and the two wound up having sex, leading a shocked Emily to call Stephanie a "brother fucker".Emily calls Stephanie and asks her for "a simple favor" to pick up Nicky from school because she has to take care of some work business, while Sean has gone to London to look after his injured mother. Stephanie obliges, but she can't get in touch with Emily. After two days, Emily hasn't come for Nicky. Stephanie calls Emily's job and is told that Emily flew to Miami to handle something, which she didn't mention to anyone, even Sean. Stephanie gets in touch with Sean to inform him, and he isn't surprised that Emily has left Nicky with her. He says that when Emily knows Nicky is with someone taking care of him, she will disappear for a few days. He returns home the next day for Nicky, and when Stephanie tells him that her assistant is unable to get in touch with Emily either, they speak to the police about Emily, who has been gone for three days. The police imply that she has left on her own because of marital troubles and Sean later tells Stephanie that Emily deleted his Facebook account once when he posted a photo of her.Stephanie takes it upon herself to find out what happened to Emily. She goes to Emily's work in New York City and meets her overbearing boss, designer Dennis Nylon (Rupert Friend), who rudely dismisses Stephanie's concerns and insults her outfit. Stephanie sneaks into Emily's office and finds a black-and-white photocopy of an unflattering photo of Emily with "Gotta have faith" written on it. Stephanie is spotted on her way out of the office and Dennis threatens her, but Stephanie remembers hearing Emily speaking brashly to Dennis, and Stephanie hits back with some sass of her own to get Dennis to back off. Stephanie then uses the photo of Emily to make missing fliers. Detective Summervile (Bashir Salahuddin) later visits Sean to inform him that Emily was not on any flights to Miami but that she rented a car from LaGuardia Airport the day she disappeared.Stephanie wraps up her original video posted after Emily had been missing for five days, updating her viewers with details about Emily's rental car. After a few days of Emily being gone, Stephanie's viewership has increased, and one of Stephanie's vlog viewers sends her a message stating that she may have spotted Emily driving in Michigan. Authorities go by a lake where the car was spotted, and sure enough, they fish the car out of the lake, as well as Emily's body. Sean is called in to identify the body, and he breaks down.A funeral is held for Emily. Nicky and Miles get into a fight when Miles says that Emily is in heaven, and Nicky yells at Stephanie, claiming that she's trying to be his new mom, and Nicky yells at his father that he is a loser as his mother said. Sean reacts angrily but Stephanie ably defuses the situation. After the reception, Stephanie and Sean console each other, which leads to the two of them having sex.Stephanie speaks with Detective Summervile and is in disbelief when told that Emily's autopsy showed that she not only had severe liver damage from alcohol abuse, but heroin in her body and track marks on her arms and between her toes from injecting heroin. She informs him that the couple fought about money like most married people do. Detective Summervile also tells her that Sean took out a $4 million life insurance policy on Emily right before she died. Stephanie insists they are looking in the wrong direction by investigating Sean, and that Dennis Nylon is way more suspicious. The detective explains that Nylon contacting them telling them that Stephanie had more information than she was revealing.Stephanie goes by the college where Sean works and sees him getting very friendly with his attractive young teaching assistant. At dinner, Nicky insists he saw his mom that day at school, and when told by Sean that he merely thinks he saw her, Nicky retorts that she spoke to him and told him to say hi to Stephanie.Growing suspicious, she asks Sean about the life insurance, which Sean says was Emily's idea after learning that Stephanie lives off her husband's insurance, so they were thinking about Nicky in case something happened to either of them. He also tells Stephanie a story about the ring that was found on Emily's finger. It belonged to his mother and was the only piece of jewelry she cared about, and although Emily claimed she had given it to her and wanted Emily to have it, Emily in fact stole it. While visiting his mother in London, the ring disappeared, and on the plane ride back to New York, Emily announced she had found it. A relieved Sean is shocked when Emily reveals she has no intention of giving it back and actually stole it, and that there was no point for his mother to take such a beautiful ring to the grave. She tells him to choose her or his mother. They would later go into the airplane bathroom where Nicky was conceived. When Stephanie tells him she knows about his threesome, he tells her that Emily was a pathological liar and that it never happened.Shortly after, Sean convinces Stephanie to move in. After Stephanie empties Emily's clothes from her large walk-in closet, she is startled to see all of Emily's items suddenly returned to their same spots. Stephanie begins to believe that Emily is a spirit who is worried about being replaced. While she is driving the boys home from school, Nicky gives Stephanie an envelope that he says is from Emily. Stephanie opens it and finds a picture of Stephanie with her family and Chris, with "brother fucker" written on it. Stephanie then receives a phone call from Emily, mocking her, and tells her to kiss Sean 4 million times for her.Stephanie thinks back to telling Emily about the day her husband, Davis, and brother, Chris, were killed. Davis confronted her on her close relationship with Chris and his affection toward Miles, and demanding to know whether Miles was his son or Chris's. She tells him he is talking crazy but does not answer the question. The two men then drive off in Davis' Camaro for a "man-to-man" talk, only to be killed. When Stephanie tearfully recounts her guilt, she and Emily end up kissing.Stephanie continues to dig into Emily's history. She takes the nude portrait of her, which she had said was painted by a former lover, and sees from the signature that it was painted by a woman. She tracks down the artist, Diana Hyland (Linda Cardellini), who claims that she was once in a romantic relationship with Emily, but her name was really Claudia. Diana was on the verge of a burgeoning career and starting to sell her work before she fell in love with her. She began painting nudes of exclusively and it ruined her career. Diana says "Emily" was a con artist who scammed her out of money after Diana paid off her student loans and college debts, and then she disappeared completely. Diana then gives Stephanie the one thing Emily left, which is a T-shirt for a Bible camp called Squaw Lake in Michigan. She warns her not to investigate Emily's past.Stephanie visits the camp and gets old books with pictures from past campers. In the book from 1996, she finds many pictures of twins Hope and Faith McLanden. After contacting numerous people with the name McLanden, Stephanie manages to find Hope and Faith's mother, Margaret (Jean Smart). Stephanie disguises herself as a cleaner and then sits down to ask Margaret about her daughters. She describes the two of them as being trouble, especially Faith. Stephanie shows Margaret the picture she found in Emily's office and confirms that the woman in the photo is Faith. It turns out that when Hope and Faith were 16, they burned down the east wing of their home with their father inside, and the two vanished without a trace.Now that she's onto something, Stephanie makes a vlog calling out to Emily, forcing her to reveal herself as being alive. She "updates" her viewers about how, to get closure, she went to Michigan to find out all about Emily's life, sharing photos from the Bible camp. She says the photos helped her feel as close as a twin to Emily, before closing with a message to Emily is, telling her that she's "gotta have Faith" wherever she is. Emily meets a shocked Sean at the restaurant they always go to on their anniversary. Emily expresses her disdain that Sean had sex with Stephanie in their home, and appears to threaten Sean with a gun, only to pull the trigger and reveal the gun wasn't loaded.Emily meets Stephanie in the cemetery by her own grave. Emily tells Stephanie her side of the story, although a flashback reveals what Emily ISN'T saying. Emily, who is really Hope, and her twin sister, Faith, burned the east wing of their house with their father inside because he was abusive. She notes that she was actually a triplet, but their sister Charity was stillborn. After the fire, they ran away from home and planned to meet up elsewhere, but Faith never showed up. After 16 years, Faith contacted Emily to meet up at the camp, where Emily found that Faith has become a heroin addict. Faith tried to blackmail Emily, and Emily drowned her in the lake and used her body to be found so that Sean and Nicky can get the life insurance money since they were broke. Back in the present, despite Emily stating that Faith's death was a suicide, Stephanie knows it was murder. Emily then accuses Stephanie of not being fully honest regarding her relationship with Chris, implying that he is Miles's real father. A flashback shows that Stephanie's husband Davis (Eric Johnson) was also suspicious as well, and that he took Chris on the car ride that ended their lives. Additionally, Emily plays a recording of her conversation with Sean in the restaurant, where he says that Stephanie meant nothing to him and that he thought of Emily during sex.Stephanie then brings in an insurance lawyer, who tells Sean that since it was discovered that Emily had a twin, the insurance company is hesitant to pay the full amount, especially since it is a large amount of money.Emily steals a wrench and tosses it into the air so that it hits her in the eye. She walks into a police station to make it look like Sean beat her and forced her to fake her death, as part of a scheme to frame Sean for insurance fraud. Sean is investigated and later taken into custody but released on bail.Stephanie then plans with Sean to get Emily to confess to the murders of her father and sister. Stephanie comes in with a gun and pretends to shoot Sean, but Emily already knew of everything and cut the mics that the police planted. She takes out her own gun and shoots Sean in the shoulder for real while also confessing to the murders. Stephanie then reveals that she has a hidden camera on her blouse, and that her viewers have all seen Emily's confession. Emily attempts to escape as Stephanie goes after her. Emily draws her gun on Stephanie, but she gets slammed by a car driven by Darren. Moments later, the police arrive to arrest Emily.Six months later, Stephanie's vlog has hit one million viewers. She continues to provide recipes and home remedies, and is now investigating cold cases.Ending text states that Stephanie's vlog continues to bring in viewers, while she has also started a detective agency and has helped the police solve at least 30 cases. Sean has written another successful book and has gotten a job working at another university, and he still lives with Nicky. Emily was sentenced to 20 years in prison, but she has adjusted to prison life nicely, as she is seen playing basketball with her fellow inmates.