A Simple Favor
Stephanie Smothers (Anna Kendrick) is a widowed single mother who runs a mom vlog with crafts and recipes. Later, Stephanie discusses the investigation into, and her feelings surrounding the disappearance of her best friend, Emily Nelson (Blake Lively). In a flashback, Emily is a busy working mother, as a PR director for a fashion company, whose son Nicky attends the same elementary school as Stephanie’s son, Miles. Emily and Stephanie become fast friends, arranging play dates for the boys and having cocktails at Emily’s house.
They trade confessions, and Stephanie admits that as a teenager, after her father died, she had sex with her halfbrother, Chris (Dustin Milligan), which Emily playfully teases her about. Emily expresses her frustration at the lack of success of her husband, Sean Townsend (Henry Golding), an English professor and writer, and their poor financial situation. One day, Emily has a work crisis and asks Stephanie to babysit Nicky after school, to which she happily obliges because Sean is in London.
After two days of Emily not responding to calls, Stephanie calls her employer, Dennis Nylon, and they tell her she is in Miami for a few days. Stephanie also calls Sean, who says Emily has done this in the past, and they call the police. While talking to the detectives, Sean mentions that Emily had no family, while she previously told Stephanie she had a sister. Stephanie mentions to Sean that she learned Emily hates having her picture taken, and Sean confirms a similar incident, but neither learn why. Stephanie goes to the Dennis Nylon headquarters and sneaks into Emily’s office and finds a photo of her, captioned with “Gotta Have Faith,” and uses it to create missing person posters.
Detective Summervile (Bashir Salahuddin) reports to Sean that Emily lied about taking a trip to Miami, and instead rented a white Kia in Michigan. A fan of Stephanie’s vlog reports seeing Emily in Michigan, and the car is discovered near a summer camp, Squaw Lake Bible Camp. The police find her drowned body in the water. Stephanie and Sean develop a bond through grief and taking care of the boys, which turns into a relationship and Stephanie moves in with him. Detective Summervile later reveals to Stephanie that Emily had severe liver damage, a large amount of heroin in her system, and that Sean had recently taken out a $4 million life insurance policy on Emily.
Emily, who is apparently alive and in hiding and is upset at the turn of events, sends Stephanie an insulting message about her secret tryst with Chris. In a flashback, Stephanie’s husband, Davis (Eric Johnson), suspected the relationship and questioned the paternity of Miles. Davis confronted Chris during a car ride, which resulted in a crash that killed them both. Stephanie begins her own investigation into Emily’s past, speaking to Diana Hyland (Linda Cardellini), an artist who painted a portrait of Emily. Diana says the girl in the painting is Claudia, her muse, and describes her as a con artist who suddenly disappeared. Stephanie asks her about Michigan, and Diana gives Stephanie an old T-shirt from a summer camp which Emily attended. Stephanie goes to the camp and discovers an old yearbook showing that Emily has a twin sister, and that their real names are Hope and Faith McLanden.
Stephanie then meets with the twins’ mother, Margaret (Jean Smart), who identifies Faith in the picture used for the missing posters. She explains that when they were 16 years old the twins had set fire to a wing of the house. Emily surprises Sean at a cafe and tells him about her plan to collect the insurance money to resolve their financial issues and become a stay at home mom, threatening to take him down with her if the plan does not work.
Meanwhile, after researching the local arson records, Stephanie is able to lure Emily into meeting her at a cemetery through cryptic messages on her vlog. Emily explains that the twins burned down the part of the mansion where their abusive father was. The two had split up while on the run to keep a low profile, and planned where to meet up after the dust settled, but Faith never appeared. Emily worked hard building a career and a life with a great family. 16 years later, Faith contacted her and asked to meet at Squaw Lake. Faith attempted to blackmail her for a million dollars and threatened to confess about the arson, taking them both down, and Emily drowned Faith in the lake. Emily lies and tells Stephanie the plan was Sean’s to use her sister’s death as an opportunity to get out of debt and flee the country.
Emily is angered by Sean’s love for Stephanie. Stephanie, likewise, is angered by Sean’s eagerness to rejoin Emily, upon discovering that she is still alive. Stephanie agrees to help Emily reappear to the public and frame Sean for physical abuse and insurance fraud. Sean is arrested, indicted, and released on bail. Stephanie has a change of heart and stages a fake argument with Sean in front of Emily to incriminate her, while policeplanted microphones are recording in the background. Emily predicts their ruse and cuts the microphones beforehand. Emily confesses to the crimes, and holds the pair at gunpoint, telling them she will kill them both and make it look like a murder-suicide. She shoots Sean in the shoulder and turns the gun on Stephanie. Stephanie then reveals that she has a hidden camera livestreaming the entire event on her vlog. Emily runs into the street, where she is hit by a car, and is then arrested.
The film concludes with text explaining that Emily was sentenced to 20 years in prison. Sean was cleared of all charges and became a successful professor at a major university. Stephanie’s vlog expanded to one million viewers, and her success in handling Emily’s case led her to become a part-time private detective.