Iris Griffith, a school psychologist at Lake Forrest Academy, an exclusive K–12 school in Atlanta, wakes on her seventh wedding anniversary beside her husband, Will, a senior software engineer at AppSec Consulting, a cybersecurity firm. They exchange their private declaration of love: "You are my very favorite person on the planet." Will prepares to leave for what he says is a three-day conference in Orlando where he is the keynote speaker, but first surprises Iris with a Cartier trinity ring symbolizing love, fidelity, and friendship: one band for each of them and "baby-to-be," since they have just begun trying for a baby. The gift is extravagant given their finances, already strained by the mortgage on their new Victorian house in Atlanta's Inman Park neighborhood.
Iris arrives late to work and learns that Liberty Airlines Flight 23, bound for Seattle from Atlanta's Hartsfield-Jackson airport, has crashed, killing all 179 passengers. She panics briefly but confirms the plane is not Will's airline or destination. Her relief vanishes when her mother calls, alarmed by reports that a William Matthew Griffith is listed among the passengers. Liberty Airlines confirms Will was on the plane. When Iris calls the conference hotel, there is no reservation under Will's name and no conference by that name. Will's assistant, Jessica, reveals she thought the couple was vacationing in Mexico and discovers Will had booked both a one-way Liberty Airlines ticket to Seattle and a Delta flight to Orlando. Delta confirms Will never checked in for the Orlando flight. The conference was a complete fabrication, backed by an elaborate flyer Will created. Iris hurls her phone across the room and retreats to bed, shaking with fury and grief.
Iris's family rallies around her: her parents; her twin brother, Dave, a Realtor; and Dave's husband, James, a doctor. Dave observes that Will has no close friends and virtually no traceable past. At the Liberty Airlines Family Assistance Center, Iris's father, a former marine, confronts the airline's Care Specialist, Ann Margaret Myers, for releasing Will's name to the media before notifying Iris. At a memorial service at the Atlanta Botanical Gardens, Iris meets Evan Sheffield, a prominent attorney who lost his wife and baby daughter in the crash and is organizing a families' association. She also meets Corban Hayes, who introduces himself as Will's gym friend, a friendship Will never mentioned. Corban tells Iris that Will had been offered a job at a Seattle tech company called ESP, which would explain his westbound flight.
At home, Iris discovers Will updated his will a month before the crash, adding life insurance policies that bring his total coverage to two and a half million dollars. His closet is suspiciously clean, as though he deliberately purged evidence. A sympathy card from "Hancock High School, Class of '99" reveals another lie: Will claimed he attended Central High in Memphis, but an online search reveals Hancock High is in Seattle. Dave finds a Hancock Wildcats T-shirt in Will's closet.
Iris and Dave fly to Seattle. ESP's head of Human Resources confirms no senior position has been open in months and she has never heard of Will. At Hancock High, yearbook photos prove Will grew up in Seattle, not Memphis. A former classmate, Coach Anthony Miller, remembers Will as "Billy Griffith," a feared bully who dealt drugs and terrorized his school. The coach reveals that Will's parents were violently abusive and his mother died in a fire, not of cancer as Will told Iris. An elderly former neighbor, Wayne Butler, confirms the fire was arson in Rainier Vista, the public housing project where Will grew up. Two young children also died alongside Will's mother. Though never proven, neighbors suspected Will set the fire, noting he appeared disappointed when firefighters carried his unconscious father out alive. Iris and Dave visit Will's father, Lewis Griffith, in a run-down memory care facility; Lewis has advanced Alzheimer's and becomes violently agitated at the mention of Billy's name. Meanwhile, Iris receives texts from a blocked number urging her to leave Seattle.
Back in Atlanta, Iris learns Will's wedding ring has been recovered from the crash site without a scratch. Then she finds a note on her bathroom counter, postmarked five days after the crash, in Will's handwriting: "I'm so sorry." She suspects Corban and meets him for coffee. When she tells him ESP never offered Will a job, Corban appears stunned and reveals that Will made him swear to look out for Iris if anything happened.
Will's boss, Nick Brackman, tells Iris that $4.47 million is missing from AppSec's accounts and all evidence points to Will. Nick threatens legal action. At the Cartier store, Iris discovers Will paid over $13,000 in cash for her ring, and the receipt lists an unfamiliar 678-area-code phone number. After calling the number and reaching only voicemail, she receives threatening texts: "Tell me where Will hid the money or you'll be joining him." Iris's father installs security upgrades, and a specialist adds a comprehensive alarm system with cameras, panic buttons, and a duress code. Texts from the blocked number continue in a caring tone, starkly different from the 678 number's threats, and Iris recognizes two different senders. A second handwritten note appears in her bathroom drawer: "Stop searching, Iris. Leave it alone. I can't protect you if you don't." When she presses the blocked number for answers, the person admits to being Will.
Iris retains Evan as her attorney. He warns that under Georgia law, if Will used stolen funds for marital assets, AppSec can hold Iris accountable. A tech expert traces the blocked number to a house in Vinings, a suburb of Atlanta. Iris drives there and sees not Will but Corban inside. The tech expert also removes a GPS tracker and spyware from her phone. Iris realizes all the texts claiming to be from Will were sent by Corban, who never once used Will's signature phrase. Reviewing court records with Evan, she learns the key witness in the arson case was a neighbor named Cornelia Huck, and the investigating officer was later discredited as corrupt, leading to the case's dismissal. The name Huck triggers a memory: Will once claimed his one remaining high-school friend went by "Huck."
Corban breaks into Iris's house one evening, bypassing her alarm with knowledge learned from Will. He reveals himself as the sender of both sets of texts and as the architect of the embezzlement: He orchestrated the theft of five million dollars in stock through a shell company in the Bahamas, with Will handling the technical execution. Enraged that Will absconded with the money, Corban takes a selfie with Iris and emails it to Will. Will responds instantly with a threat, then calls Iris's phone and tells her to get out, confirming he is alive. Evan, who had returned for his forgotten wallet and sensed something wrong, calls the police. Officers surround the house. A single bullet pierces the window and kills Corban. No officer claims to have fired the shot. Iris runs into the backyard screaming Will's name, certain he fired the bullet. He is already gone. Evan cautions that because Corban was unarmed, the killing is legally classifiable as murder, meaning Will is now wanted for more than embezzlement.
Police discover the missing AppSec money planted on Corban's computer, evidence Will placed to frame Corban. Iris tells the detective the full truth: She spoke to Will on the phone, believes he is alive, and believes he shot Corban. Without hard evidence, the detective cannot act. Iris puts her house on the market, unable to afford the mortgage alone, and moves Will's father to a better care facility. She and Evan develop a close friendship, though both acknowledge it is too soon for anything more.
On day 33, Will appears in Iris's kitchen, thinner, grayer, and visibly worn. Iris kisses him, then slaps him hard across the face twice. Will explains: Corban's real surname is Huck. He is Cornelia Huck's son and Will's childhood acquaintance from Rainier Vista, who concealed his identity behind the alias Corban Hayes, tracked Will down in Atlanta, and blackmailed him into executing the stock transfer by threatening to expose Will's past to Iris. Will confirms that Corban, not he, set the fire that killed his mother and the two children, though Will admits he knew beforehand and did nothing. He says he faked his death not for the money but to preserve Iris's image of the man she married. He asks Iris to disappear with him.
Iris refuses to leave her family. They go upstairs and make love one final time. In the middle of it, Iris reaches behind her nightstand and holds the panic button for three seconds, summoning the police to arrest her husband.