Christopher Robin is leaving for boarding school, so his friends from the Hundred Acre Wood – Winnie the Pooh, Tigger, Piglet, Eeyore, Kanga, Roo, Owl, and Rabbit – throw a goodbye party. Christopher comforts Pooh and tells him that he will never forget him.
Christopher goes to boarding school but gets reprimanded for drawing pictures of Pooh during class. Soon after this, his father dies, forcing Christopher to grow up. After entering adulthood, Christopher moves to London, meets and marries an architect named Evelyn, with whom he has a daughter named Madeline. He serves in the British Army during World War II. After the war, he works as an efficiency expert at Winslow Luggages. He neglects his family due to his demanding job and plans on sending Madeline to boarding school as well. With the company hitting hard times, Christopher's boss, Giles Winslow tells Christopher to decrease expenditures by 20%, largely by choosing which employees to lay off, and to present his plan on Monday. This causes Christopher to reluctantly miss joining his family at their countryside cottage in Sussex for a summer-ending weekend.
Meanwhile, Pooh awakens and is unable to find his friends, so he decides to travel through Christopher's door and finds himself in London. He reunites with Christopher, who is shocked to see Pooh, but takes him back to his home. After a night and morning of chaos (including Pooh knocking down all of his kitchen shelves in search of honey), Christopher escorts Pooh back to Sussex on the next train, bringing his work papers along with him.
After sneaking past Christopher's cottage, the two enter the Hundred Acre Wood. Christopher becomes exasperated by Pooh's absent-mindedness and fear of Heffalumps and Woozles, and angrily tells Pooh that he is not a child anymore, before the two get separated in the fog. After falling into a "Heffalump Trap", which is then flooded by rainfall, soaking him and his belongings, Christopher discovers Eeyore and Piglet, who lead him to the others, hiding in a log out of fear of a Heffalump. Because of his attempts to convince his friends of the nonexistence of monsters leaves him unable to persuade them that he is truly Christopher Robin (Roo retorts that "the real Christopher Robin" would defeat the Heffalump), he and Eeyore fake the defeat of the Heffalump to convince the others. Finally realizing that he is Christopher Robin, they joyfully greet him. When they reunite with Pooh at their meeting spot, Christopher apologizes for getting upset earlier. Christopher tells Pooh how lost he feels, but Pooh reminds him that they have found each other and comforts him with a hug.
The next morning, Christopher, realizing how late he is, rushes from the Hundred Acre Wood to make his presentation in London. He encounters his family and, unwilling to tell them about his old friends in the Hundred Acre Wood, leaves abruptly, upsetting Madeline. Pooh discovers that Tigger removed Christopher's work papers when drying his briefcase, so Pooh, Tigger, Piglet and Eeyore decide to travel to London to give it back. They meet Madeline, who recognizes them from her father's drawings. She joins them, wanting to dissuade her father about boarding school; Evelyn follows after discovering a note Madeline left.
At the meeting, Christopher discovers that his papers are not in his briefcase, which instead contains objects from the woods that Tigger left for him, including Eeyore's detachable tail. Evelyn arrives and Christopher joins her to search for Madeline. Madeline, Pooh and the others stow away in a truck carrying Winslow company crates, but Tigger, Eeyore and Piglet are accidentally thrown out, and they encounter Christopher and Evelyn in the process. Pooh and Madeline arrive near the Winslow building and reunite with Christopher, Evelyn and the others, but Madeline accidentally trips on the stairs and loses all but one of the papers, upsetting her and Pooh. Christopher assures Madeline of her importance to him and that he will not send her to boarding school. He does get an idea from the paper while Evelyn is approaching.
With his family and friends in tow, Christopher improvises a new business plan involving reducing the prices of luggage, giving employees paid leave, and selling their luggage to everyday people instead of just the wealthy to increase demand. Winslow dismisses the idea, but his father warms to it and agrees to the plan. Winslow is humiliated when it is discovered that he contributed nothing to the plan, having been golfing all weekend. Christopher later takes his family into the Hundred Acre Wood with Pooh, Eeyore, Piglet and Tigger to meet the rest of his friends. While Evelyn and Madeline have tea and fun with most of Christopher's friends, he and Pooh share a tender moment at their special spot together.
