Blix Holliday, an 85-year-old woman living in Brooklyn with her partner Houndy, a lobsterman, reluctantly attends her niece Wendy Spinnaker's annual post-Christmas tea in Fairlane, Virginia. Blix is concealing a terminal abdominal tumor she has named Cassandra and came at Houndy's urging to see her family one last time. Long considered the family misfit after being cheated out of the homestead that passed to Wendy, Blix feels an instant recognition when her grandnephew Noah arrives with his fiancée, Marnie MacGraw, a nursery school aide from Florida. When Marnie declines Welsh rarebit, mistaking the cheese dish for rabbit meat, Wendy humiliates her publicly while Noah plays pool with his best friend, Simon Whipple. Blix rescues Marnie, and the two discover they share a matchmaking instinct. Blix tells Marnie she is "in line for a big, big life." After the couple departs, Blix senses with certainty that they will not marry.
Five months later, Noah arrives nearly an hour late to the wedding in Jacksonville, Florida, takes Marnie outside, and tells her he cannot go through with it, citing a fear of domesticity. Marnie talks him through his panic, and they walk down the aisle together, he in jeans, she in a grass-stained dress. At the reception, Blix gives Marnie a mantra: "Whatever happens, love that." The marriage collapses on the honeymoon in Costa Rica when Noah reveals he secretly applied for a fellowship in Africa with Whipple, accepted a week before the wedding, and admits he does not want the house, children, or teaching career they planned. Back in California, Noah moves out, and Marnie signs online divorce papers.
Marnie spirals into denial at work, pretending she is still married, until she arrives one morning in her bathrobe carrying her wedding dress. She loses her job, and her parents take her home to Jacksonville.
Meanwhile, Blix tends to her Brooklyn community. Her circle includes Lola Dunleavy, her best friend and a longtime widow who lives next door; Jessica, a single mother upstairs whose ex-husband Andrew cheated on her; Jessica's 10-year-old son Sammy; and Patrick Delaney, a reclusive burn survivor and former sculptor in the basement. One morning, Blix realizes Patrick needs Marnie. They are a match.
Blix plans a farewell party she calls a "Blix Out." She persuades Patrick to attend by placing his hand on her tumor, telling him they are both broken and must still dance. During the celebration, Houndy collapses and dies of a sudden cardiac event. Blix survives into autumn, when Noah returns from Africa after his fellowship fell apart. In Blix's final days, Patrick and Noah sit with her. Patrick understands her wish to die at home and gently overrules Noah, who pressures her to seek medical care. Patrick holds her hand as she dies.
In Jacksonville, Marnie reconnects with Jeremy Sanders, her old high school boyfriend, now a physical therapist. Their comfortable, family-endorsed relationship deepens, and Jeremy proposes. Marnie says yes. Shortly after, a letter from a Brooklyn law firm informs her that Blix has left her the brownstone. Marnie flies to New York, planning to sell the property and return.
At the house, she discovers Noah already living there. Blix's attorney, Charles Sanford, reveals the will's stipulation: Marnie must live in the house for three months before it becomes hers. If she refuses, the property goes to charities. Marnie opens a private letter from Blix, who explains that the idea came right after Marnie called asking for a spell to get Noah back. Blix expresses her conviction that the house will transform Marnie's life. Moved by the letter, Marnie opens a bank account and decides to stay.
Noah persuades her to let him remain as a roommate. Marnie integrates into the community: Jessica introduces her to the neighborhood; she develops a warm friendship with Patrick through texts and visits; and Dorothy, the owner of a flower shop called Best Buds, hires her. A golden haze settles over Marnie's perception as she senses love everywhere and feels Blix's presence.
Marnie and Noah resume a sexual relationship while she maintains her engagement to Jeremy through daily phone calls. She grows suspicious of Noah when he admits his mother asked him to search Blix's letter for evidence of manipulation. Opening Blix's spell book, Marnie finds a hidden journal containing a note dated the day before Blix died: "MARNIE NOAH HAS TO LEAVE DO NOT LET HIM STAY!!" On the journal's last page, Blix listed her matchmaking projects, with "PATRICK AND MARNIE" written four times. Stunned, Marnie asks Noah to leave. She also begins performing spells from the book, including one to encourage Jessica and Andrew's reconciliation.
Patrick overhears Noah on speakerphone with Wendy, who wants evidence that Blix was mentally unsound to contest the will. Noah has been secretly removing Blix's artwork and papers and shipping them to Virginia. When Marnie discovers her letter from Blix has been stolen, Patrick reveals he intercepted the boxes, keeping everything safe.
Over Thanksgiving preparations, Patrick tells Marnie his story. Four years earlier, a gas leak caused an explosion in the studio he shared with his partner, Anneliese. Patrick ran toward the fire, sustaining severe burns to his face and hands. She died. After 13 surgeries, he retreated into isolation until Blix found him and convinced him to move into her basement. Marnie kisses his scarred cheek. He jerks away, insisting he cannot stand pity. She tells him she loves him. He does not respond.
Thanksgiving dinner becomes a catastrophe. Noah appears uninvited, having brought Jeremy from Florida as a surprise orchestrated by Marnie's family. When Marnie sees Jeremy, she drops the turkey. Bedford, the household dog, drags the carcass into the living room. Jeremy confronts Marnie on the stoop, realizes she slept with Noah, and walks away. Patrick comes upstairs, tends to Marnie, and puts her to bed in his apartment, sleeping beside her.
In the aftermath, Jessica and Andrew begin reconciling. Noah confesses he stayed to monitor the house for Wendy, who fears Blix's papers could expose the family. He departs for Bali. William Sullivan, Lola's persistent suitor, announces "A Year of One Hundred Dates with Lola," a patient campaign of companionship at whatever pace she wants.
Brooklyn's first snowfall brings the turning point. While Marnie and Sammy sled in Prospect Park, Bedford escapes and is hit by a car. Patrick rushes outside from his basement, something he almost never does, and drives the dog to the animal hospital. He then runs through the snow to find Marnie. As they walk together, he takes her hands and says he does not want to live without her. They kiss in the snow.
On the day the three months expire, Marnie signs the final papers and receives Blix's second letter, which welcomes her to her "big, big life" and tells her to keep loving Patrick, whom Blix describes as damaged but luminous with trapped light. In an epilogue one year later, Patrick has moved in with Marnie and begun painting Brooklyn sunsets. At a store, a small girl touches his scars and asks if they hurt. He smiles and says, "No. No more hurt. Not anymore." Marnie works at Best Buds with Blix's spell book on the counter, and she and Patrick bake cupcakes bearing a single message: "WHATEVER HAPPENS, LOVE THAT."