The story begins with Beatrix Potter nervously packing her portfolio and narrating that she is a London spinster, and that her ambition to become a children’s author meets with wide disapproval. She and her chaperone, Miss Wiggin, visit the publishing house of the Warne brothers and they decide to publish her book. Beatrix is thrilled and returns home, not without taking a drive through the parks to celebrate first. However it is revealed that the Warne brothers think her book is ridiculous and will no doubt be a failure. The only reason they agreed to publish her story is because they promised their youngest brother, Norman, a project.
When Norman Warne arrives, Beatrix makes decisions about her finished book, regarding size, colour and price. Norman admits he has never done anything like this before but has given her book a great deal of thought. Beatrix realises what the Warne brothers have done regarding her and Norman but they become determined to prove them wrong. Norman takes Beatrix to the printer, and she has her drawings reproduced and copies of her book sold. Thrilled, Beatrix and Norman visit the Warne family, where Beatrix meets the wheelchair-bound but lovely Mrs. Warne, and Norman’s sister, Amelia, nicknamed “Millie”. Millie has decided that she and Beatrix are going to be friends and is overjoyed that Beatrix is a spinster, as is Millie, who believes men to be nothing but bores. The family befriends Beatrix, yet Helen Potter, Beatrix’s social-climbing mother, is unhappy about her daughter spending time in the company of tradesmen.
When she returns home, Beatrix and Helen bicker about Beatrix’s stubborn decision not to marry. Beatrix reminds her mother of the book she wrote, and her mother retorts she believes the venture will fail. However, the book sales are very successful and copies are displayed in many store windows. Even Beatrix’s father, Rupert, buys a copy along with friends at the Reform Club. Encouraged by this success and her father’s support, Beatrix invites Norman and Millie to her family’s Christmas party, despite her mother’s misgivings. At the party everyone enjoys themselves and Beatrix shows Norman a story she is writing especially for him, “The Rabbits’ Christmas Party”. She shows him a drawing from the story and shows him her studio where she writes and draws. Miss Wiggin falls asleep from too much brandy (a generous portion of which had been added to her coffee cup by Norman), and Norman plucks up the courage to propose to Beatrix. Mrs. Potter interrupts before Beatrix can reply, and they join the other guests in the drawing room. Beatrix confides in Millie about Norman proposing, and Millie encourages her to say yes. Beatrix then tells the guests of the stories she writes and they are delighted and amused. Mrs. Potter, however, can not see what all the fuss is about. As the guests leave, Beatrix whispers her agreement to marry Norman, who is overjoyed.
Soon after, Rupert Potter invites Norman to his office and tells him his opinion of him. At the Potter household, Beatrix and her parents argue about her decision to marry Norman. Beatrix is adamant and will not be dissuaded. Mrs. Potter tells her no Potter can marry into trade, but Beatrix reminds her that her grandfathers were both tradesmen. When Mrs. Potter threatens to cut her daughter off, Beatrix reminds them of her brother, Bertram, who married a wine merchant’s daughter and was not disowned. She states she can survive on her own with her novels. Mr. Potter attempts to reason with his daughter, but she tells him she wants to be loved and not simply marry someone because he can provide for her.
Beatrix inquires with the bank about her royalty earnings, wondering if she would perhaps someday be able to buy a house in the country. She is amazed and delighted to learn that her book sales have made her wealthy enough to buy several estates and a house in town if she wishes.
When she returns home her parents offer a proposition: that Beatrix keep her engagement to Norman a secret and vacation with them in the Lake District for the summer. If she still wishes to marry him at the end of the summer, they agree that they will not object to the marriage. Beatrix agrees to the proposition and is quite convinced that she will not change her mind, telling her parents to prepare for an October wedding.
Norman and Beatrix kiss each other goodbye at the train station and write many letters during their time apart, until one day a letter arrives from his sister Millie, informing her that Norman is ill. Beatrix travels back to London only to find that Norman has died. Overcome with grief, Beatrix shuts herself up in her room. She turns to her drawing, but discovers that her characters disappear off the page. Millie comes to visit and comfort her, and Beatrix decides she must leave the house.
Beatrix buys a farm in the country in the Lake District and moves there to resume her work. She hires a farmhand to run the farm and finds comfort in her surroundings. With the help of her solicitor, William Heelis, she outbids developers at auctions and buys many other farms and land in the area to preserve nature. Eight years after moving to the Lake District she marries William. The land eventually forms part of the Lake District National Park in northwestern England.