The film unfolds as Beatrix Potter, played by Renée Zellweger, finds herself filled with anxiety while packing her portfolio. She narrates her story as a London spinster whose aspirations to become a children's author are met with widespread skepticism. Accompanied by her chaperone, Miss Wiggin, Beatrix visits the publishing house of the Warne brothers, where they surprisingly decide to publish her book. Overwhelmed with excitement, she takes a celebratory drive through the parks before returning home. However, it's soon revealed that the Warne brothers deem her book to be absurd and destined for failure, only agreeing to publish it to honor a promise made to their youngest sibling, Norman Warne, portrayed by Ewan McGregor.
Upon Norman's arrival, Beatrix engages in discussions about critical decisions regarding her book's final details—size, color, and price. Although Norman confesses he's never tackled a project like this before, he has thoughtfully considered Beatrix's work. Together, they become increasingly determined to prove the Warne brothers wrong. Norman takes Beatrix to the printer, where her artistic illustrations come to life as copies of her book are sold. Filled with joy, the duo visits the Warne family, where Beatrix encounters the wonderful Mrs. Warne, confined to a wheelchair, and Norman's cheerful sister, Amelia, affectionately known as "Millie."
Millie, who is thrilled to discover that Beatrix is also a spinster, believes that men are simply tedious and decides that they will be friends. While the Warne family welcomes Beatrix with open arms, her mother, Helen Potter, is less than thrilled about her daughter mingling with tradespeople. The tension escalates upon Beatrix's return home, where she and Helen clash over Beatrix's strong refusal to conform to societal expectations of marriage. Beatrix insists on the importance of her book, to which Helen dismissively predicts its inevitable failure. Contrary to her mother’s beliefs, the book enjoys tremendous success, adorning window displays across various stores. Even Beatrix's father, Rupert, joins her friends in purchasing a copy at the Reform Club. Empowered by this achievement and her father's enthusiastic support, Beatrix invites Norman and Millie to join her family for Christmas, despite her mother's objections.
The Christmas gathering is filled with merriment, and Beatrix shares a story she's crafting for Norman, titled "The Rabbits' Christmas Party." She excitedly reveals her illustrations and introduces him to her creative space. After a bit too much brandy leaves Miss Wiggin dozing off, Norman gathers the courage to propose to Beatrix. Just as she is about to respond, Mrs. Potter interrupts, and they head to the drawing room to join other guests. Beatrix confides her mixed feelings about Norman's proposal to Millie, who encourages her to accept. During the event, Beatrix captivates the guests with tales from her stories and delights in their reactions, while Mrs. Potter remains oblivious to the excitement surrounding her daughter's work. As the evening draws to a close, Beatrix quietly whispers her acceptance of Norman's proposal, bringing him immense joy.
Shortly after, Rupert Potter arranges a meeting with Norman to express his personal views. Meanwhile, tensions arise at the Potter household over Beatrix's willingness to marry Norman, as she stands resolute in her decision. Mrs. Potter sternly opposes the idea of her daughter marrying someone from the trade, yet Beatrix counters that both her grandfathers were tradesmen. Threatened with financial disowning, Beatrix retorts with evidence from her brother Bertram's marriage to a wine merchant's daughter, asserting her independence through her literary success. She inquires about her royalty earnings at the bank and is taken aback by the revelation that she has amassed enough wealth to buy multiple estates and a city home if she desires.
Upon returning home, Beatrix's parents propose that she keep her engagement a secret while they vacation in the Lake District for the summer. They agree that if she still wishes to wed Norman after the summer, they would no longer oppose the marriage. Beatrix accepts their terms, confidently announcing plans for an October wedding. At the train station, she and Norman share a heartfelt goodbye, exchanging letters filled with affection during their time apart. Tragically, Beatrix soon receives a letter from Millie, revealing that Norman has fallen ill. She rushes back to London only to discover the heartbreaking news of his death. Overwhelmed with sorrow, Beatrix retreats into her room, struggling to find comfort in her art as her characters seem to fade away.
Millie visits to provide solace, motivating Beatrix to reclaim her life outside the walls of her grief. Determined to start anew, Beatrix acquires a farm in the picturesque Lake District, hiring a farmhand to help manage it. She finds peace within her surroundings and, with the guidance of her solicitor William Heelis, diligently outbids developers at auctions to obtain and preserve the natural beauty of the land. Eight years after her move, Beatrix marries William, and her lands contribute to the creation of the Lake District National Park in northwestern England.
