Seven years before her youngest daughter's wedding, Beth Carmichael died of ovarian cancer. In the months before her death, she filled a spiral-bound notebook with detailed instructions, memories, and advice for the wedding she knew she would never attend. The Notebook guides nearly every aspect of the celebration and becomes a sacred family document.
Jenna Carmichael is marrying Stuart Graham on Nantucket Island, the family's longtime summer home. Jenna's older sister Margot, eleven years her senior and the maid of honor, travels to the island by ferry with Jenna, Jenna's best friend and bridesmaid Finn Sullivan-Walker, and Margot's three young children. Margot works at an executive search firm and is divorced from Drum Bain Sr., who has just announced his engagement to a Pilates instructor. She is secretly involved with Edge Desvesnes, her father's fifty-nine-year-old law partner, hiding the affair because Edge insists her father, Doug Carmichael, would be devastated. On the ferry, Margot encounters Griffin Wheatley, a man she once wronged professionally, and the surprise meeting unsettles her.
Upon arriving at the Nantucket house, Jenna discovers the Notebook is missing. Back in Connecticut, Doug finds it on his kitchen counter and calls to reassure the family. He then confronts his second wife, Pauline, who admits she took the Notebook from a restaurant and read it out of jealousy over Beth's hold on the family. Doug realizes he does not love Pauline and resolves to ask for a divorce after the wedding. He tells Pauline he does not want her at the wedding, but she insists on coming.
Thursday evening brings the first of many crises. Roger, the wedding planner, informs Margot that a branch of Alfie, the family's beloved tree, must be cut to accommodate the reception tent. Margot reluctantly authorizes the cut. At the bachelorette dinner, Finn leaves in tears over her husband Scott's absence, and Jenna follows her home, abandoning her own party. Margot salvages the evening with the remaining bridesmaids: Autumn Donahue, Jenna's college roommate, and Rhonda Tonelli, Pauline's daughter. She confesses her affair with Edge and immediately regrets it. Later, she drops her phone in a toilet, losing messages from Edge, and encounters Griff again at a bar but cannot accept his company because of her guilt.
On Friday, Jenna's brother Kevin Carmichael, a structural engineer, devises a way to lift Alfie's branch with ropes and a cherry picker instead of cutting it, saving the tree. Margot resents Kevin's effortless heroism after months of managing the wedding logistics herself. Doug confides to Margot that he plans to divorce Pauline. At the golf course, Margot runs into Griff for the third time but declines his invitation.
Stuart's mother, Ann Graham, a six-term North Carolina state senator, arrives carrying deep anxiety about one guest: Helen Oppenheimer. Years earlier, Ann's husband, Jim Graham, had an affair with Helen. Helen became pregnant with Chance, Stuart's half brother. Jim left Ann, married Helen, then eventually returned to Ann, and they remarried. Ann invited Helen to the wedding to prove she had moved past the betrayal, never expecting Helen to accept.
At the rehearsal, Doug walks Jenna down the aisle to Pachelbel's Canon, the piece Beth loved, and breaks down weeping. Jenna's brother Nick and Finn miss the rehearsal after paddleboarding together, raising Margot's suspicions about their closeness. At the rehearsal dinner, Chance has a severe allergic reaction and is rushed to the hospital; Jim and Helen follow together, leaving Ann behind. That evening, Helen reveals to Jenna that Stuart was previously engaged to a woman named Crissy Pine for five weeks. Jenna, blindsided by Stuart's concealment, declares the wedding off.
Margot finds Jenna crying on the front steps and tries to change her mind, but Jenna is immovable, citing every failed relationship around her as proof that love dies. Ann and Jim also fight that night; Ann orders Jim out of their hotel room, and he sleeps in the rental car.
Seeking escape, Margot meets Griff at a bar. They share a deep conversation about loss, and he walks her home. They kiss on the sidewalk, but Margot pulls away, knowing she cannot pursue him because of what she did.
Saturday morning, Margot wakes to find Jenna gone. She searches the three places where Beth's ashes were scattered: Brant Point Lighthouse, Madaket Beach, and the tower of the Congregational Church. At the church tower, Jenna delivers an anguished speech about witnessing their mother's death while her older siblings were far away and about watching love fail everywhere she looks. Margot hands her a phone and tells her to call Roger to confirm the cancellation. Instead, Jenna dials Stuart and tells him she loves him. The wedding is back on.
That afternoon, Margot discovers Jenna's wedding band is missing and traces it to her six-year-old daughter Ellie, who hid it in a change purse. Doug visits Jenna in the attic, where she stands in Beth's vintage Priscilla of Boston wedding gown. Overcome, he struggles to contain his emotions as Jenna pins his boutonniere and straightens his tie, fulfilling Beth's instructions.
At the ceremony, Margot spots Edge seated with Rosalie Fitzsimmon, his young paralegal, whom he brought as his date without warning. Pauline runs sobbing from the church during the readings. Helen inserts herself into the recessional by processing out ahead of Ann and Jim, enraging Ann.
At the reception, the tent realizes Beth's vision of an elegant woodland in white and green. Edge tells Margot he has been seeing Rosalie since January and that their relationship was never going to progress. During a dance with Doug, Margot confesses everything: the affair with Edge, and how Edge manipulated her into sabotaging a job candidate to benefit his nephew. When Margot says she does not believe in love, Doug cites his marriage to Beth as proof that it exists. He confronts Edge outside the tent and expels him from the wedding.
That evening, Pauline demands a divorce, preempting Doug's own plan. In a final act of rage, she throws the Notebook into the bonfire, and Doug watches Beth's words burn. Jenna later reveals that Stuart scanned the entire Notebook into his computer, preserving it digitally.
Margot seeks out Griff and confesses she sabotaged his candidacy for a position at Tricom by weaponizing a detail from his résumé so Edge's nephew would get the job. Griff is stunned and walks away.
Sunday morning, Ann hosts a southern-themed brunch. Jim gives Ann a pearl choker made with the diamond from her original engagement ring, symbolizing their renewal. Ann confronts Helen, saying she cannot make peace with what Helen did. Scott Walker arrives to reclaim Finn; Jenna reveals she called him. Pauline has departed Nantucket alone, and Doug announces he will spend the rest of the summer on the island.
On the ferry home, Margot throws pennies overboard at Brant Point Lighthouse, honoring Beth's tradition of ensuring a return to Nantucket. Griff appears on the upper deck. He tells Margot he is happy at the company where he landed and that he understands why she confessed: not because she had to, but because she is a good person. He asks her to call him that night. As Griff walks away, he says he cannot wait to come back to Nantucket. Margot, looking at the island that holds her mother's spirit, agrees.
The Notebook's final page, titled "Happily Ever After," contains Beth's reflection that no one can guarantee a happy marriage. She credits Doug with two things he did every day: He made her laugh, and he was her friend. She closes with gratitude: "How lucky, how very lucky, I have been" (398).