Luke Richardson, a cell phone engineer in Farmington Hills, Michigan, returns home from his wife Natalie's funeral with his three children: Will, 14; May, 9; and Clayton, 3. Natalie died of Ewing's sarcoma, a rare soft tissue cancer, and Luke is numb with grief. As he carries sleeping Clayton upstairs, he slips on a blue envelope addressed to him in Natalie's handwriting. The letter, dated from her first day of chemotherapy, is warm and playful, promising more letters and asking Luke not to share them with the children. For the first time in months, he feels something like hope.
A second letter arrives the next morning with a recipe for Natalie's special pancakes. Annie Gurrella, Natalie's best friend, shows up to help as she has every morning for months and recognizes Natalie's handwriting, but Luke insists the letters are private. When May sees her chocolate chip smiley-face pancake, she becomes convinced her mother has returned, and Luke must gently tell her Natalie is dead. Annie comforts the devastated girl while Luke breaks down sobbing for the first time.
Over the following weeks, more blue envelopes arrive at irregular intervals, narrating Natalie's treatment and giving instructions: return to work, get the kids back to school, find routine. Luke obeys. Annie begins watching Clayton at her house, where her husband, Brian Gurrella, a local police officer, works nights. Annie warns Luke that someone must be delivering the letters and urges him to investigate, but he refuses. A later letter acknowledges Natalie will not survive and urges Luke to hire Jessie Fraga, an education student at Eastern Michigan University who has lived with chronic kidney disease since childhood. Luke hires Jessie, who arrives carrying her own blue envelope from Natalie.
A call from Will's guidance counselor, Felicity Mason, reveals that Will believes he is adopted after finding an envelope from Maranatha Adoptions in Chicago among Natalie's belongings. Luke reassures him but agrees to investigate. On the back of the envelope, he notices the name "Dr. Neal," which also appears in Natalie's phone contacts. As Natalie's letters increasingly mention Dr. Neal, her methods professor at Eastern Michigan, Luke grows suspicious. He finds a remaining text from Dr. Neal on Natalie's phone: "I'm glad we found each other," with all prior messages deleted. Annie recalls meeting a "Pastor Neal" at Natalie's bedside but dismisses Luke's fears of an affair.
In April, Luke visits the Maranatha Family Services maternity house in Kalamazoo, a crisis pregnancy center for young women who have chosen adoption. The director refuses to release information, but Luke spots a photograph showing Natalie and Andy Garner, her high school boyfriend, in matching Maranatha T-shirts. He suspects Natalie may have had a child with Andy.
At May's birthday party, Luke observes bruises on Annie and suspects Brian is abusing her, a fear rooted in his own childhood with a father who was violent and had an alcohol addiction. Annie privately reveals she does not want to move for Brian's new job in Washington, DC, because it would mean breaking her deathbed promise to Natalie to help care for Luke and the children. Following Natalie's instructions, Luke begins dating Felicity. Before the children depart for six weeks in Florida, Natalie's mother, Terry, accuses Luke of having an affair with Annie and warns he will end up like his father, a devastating reference to his abusive upbringing.
Over the Fourth of July weekend, Luke and Annie drive to Pentwater to confront Andy at Luke's childhood home. Andy reveals he never had a child with Natalie. His daughter was born to a classmate, and Natalie connected him to Maranatha House through her pastor, Neal Townsend. Luke now knows Dr. Neal and Pastor Neal are the same person.
During a date with Felicity, Luke receives frantic texts from Annie and rushes to her house. He finds broken glass and Annie huddled on the stairs after Brian destroyed the house in a rage. Brian is passed out in the bedroom surrounded by bottles and pills. Luke takes Annie home with him. That night, he tells Annie about his childhood for the first time: his father's violence, the stillbirth of his baby sister Violet after his mother was beaten, and his mother's death from hemorrhaging that same night. Luke was fourteen, hiding in a shed in Natalie's backyard, when police found him and placed him in foster care. Annie returns to Brian the next morning, and Luke tells her not to contact the family until she is ready to leave.
On his wedding anniversary, a florist's invoice reveals that "N. Townsend" has been sending the letters and flowers. Luke hurls the delivery across the room. While packing Natalie's clothes, he discovers a hidden scrapbook behind a loose baseboard, filled with clippings about Mallory Witling, a three-year-old from Lansing, Michigan, who disappeared and was presumed dead. Mallory's adoptive mother, Eva Witling, had Munchausen syndrome by proxy, a disorder in which a caregiver deliberately makes a child sick, and had poisoned both Mallory and an older daughter. A letter hidden in the scrapbook reveals that Mallory was Natalie's biological daughter, placed through Maranatha House. Luke calculates the timeline: Natalie was fourteen when the baby was born, conceived the day his mother and sister died, when he and Natalie had sex for the first time.
After the children return from Florida, Jessie collapses into kidney failure while babysitting. Her medical alert bracelet reads "JESSIE TOWNSEND," with Neal Townsend as emergency contact. On his way to the hospital, Luke is arrested after Brian plants pills in his car, retaliating because Luke disclosed Brian's alcohol use to a job investigator, costing Brian the position. Annie compiles evidence of Brian's violence and drug theft. Brian confesses to planting the drugs, all charges against Luke are dropped, and Brian is arrested.
At the hospital, Luke confronts Neal at Jessie's bedside. Neal insists his relationship with Natalie was pastoral, never romantic. He gives Luke a final letter marked "The End," dictated by Natalie because she was too weak to write. Natalie reveals the full truth: She gave birth to their daughter at fourteen and placed her through Maranatha House, where Neal and his wife Maria facilitated the adoption. She never told Luke because he had said he never wanted children, fearing he would become like his father, and because she was frightened after he struck her in the shed. She later learned their daughter had been adopted by the Witlings and was presumed dead. Neal then reveals the most stunning truth: Their daughter is alive. Mallory's adoptive father, Mark Witling, brought the poisoned toddler to Maranatha after discovering his wife was harming the child, and Neal and Maria raised the girl as their own, renaming her Jessie. The ethylene glycol poisoning caused the chronic kidney disease Jessie has lived with her entire life.
Terry, overhearing the revelation, recognizes Jessie as the granddaughter she had secretly been trying to find and volunteers as Jessie's kidney donor. Before surgery, Luke tells Jessie that Natalie was her birth mother and he is her birth father. Jessie asks if Natalie might be waiting for her in heaven. Luke, a lifelong nonbeliever, tells her that if there is any way for Natalie to find her, she will. He brings a shoebox containing all 58 of Natalie's letters to give Jessie after surgery, fulfilling Natalie's final wish. As Jessie enters the operating room, Luke resolves that there will be no more waiting. Tomorrow, they will start living again.