The film is based on three stories found in the Milne books. Two stories are from Winnie-the-Pooh: “In Which Eeyore Loses a Tail and Pooh Finds One” and “In Which Piglet Meets a Heffalump”. The other story is found in The House at Pooh Corner: “In Which Rabbit Has a Busy Day and We Learn What Christopher Robin Does in the Mornings”. Some elements, such as the gang thinking that Christopher Robin has been captured by a monster, are based on events from the film Pooh’s Grand Adventure: The Search for Christopher Robin.
Pooh wakes up one day to find that he is out of honey. While out searching for more, Pooh discovers that Eeyore has lost his tail. Pooh, Piglet, Rabbit, Owl, Kanga, and Roo come to the rescue while Tigger has his bouncing fun, and Christopher Robin decides to hold a contest to see who can find a replacement for Eeyore’s tail. The prize for the winner is a fresh pot of honey. After many failed attempts for what would replace Eeyore’s tail (such as a cuckoo clock), Kanga suggests they use a scarf, but it unravels.
The next day, Pooh goes to visit Christopher Robin and he finds a note that says “Gon Out Bizy Back Soon”. Because Pooh is unable to read the note, he asks for Owl’s help. Owl’s poor reading comprehension skills lead Pooh and his friends to believe that Christopher Robin has been abducted by a ruthless and mischievous monster they call the “Backson” and Owl describes it in a song that is shown in a chalk-drawn scene. Rabbit plans to trap the Backson in a pit, which they think he’ll fall into after following a trail of items leading to it. Meanwhile, Tigger, wanting a sidekick to help him defeat the Backson, recruits Eeyore to be a second Tigger. He dresses up like the Backson and tries to teach Eeyore how to fight. Eeyore, who is doing this against his will, escapes from Tigger and hides underwater.
After a failed attempt to get honey from a bee hive, Pooh’s imagination combined with his hunger get the better of him which has end up eating some mud and later, accidentally falls into the pit meant for the Backson. Rabbit, Kanga, Roo, Owl, Piglet, and Eeyore (who had found an anchor whilst he was hiding to replace his own tail) try to get him out, but fall in themselves. Piglet, who didn’t fell in, attempts to get Pooh and friends out of the trap (though continuously irritating Rabbit with over-interpretations of his instructions), but he runs into Tigger, still in his Backson outfit, and mistakes him for the actual monster. Piglet escapes from Tigger on a red balloon, which knocks some of the storybook’s letters into the pit. After the chase, Tigger and Piglet fall into the trap as well, where Eeyore reminds Tigger that he, being “the only one,” is “the most wonderful thing about Tiggers”. Eventually, Pooh figures out to use the fallen letters to form a ladder, and the animals are able to escape the pit. They soon find Christopher Robin, and tell him about the Backson, but he clarifies, saying he meant to be “back soon.” The hunny pot prize was given to the red balloon from earlier, much to Pooh’s dismay.
Later, Pooh visits Owl only to find that Owl was the one that took Eeyore’s tail, not realizing it belonged to Eeyore. Owl had been using Eeyore’s tail as a bell-pull for his door. Pooh chooses to leave and return the tail to Eeyore instead of sharing a pot of honey with Owl. Christopher Robin is proud of Pooh’s selflessness and rewards him with a large pot of honey.
In a post-credits scene, it is revealed that the rumored Backson actually exists deep in the woods, but is much friendlier than imagined. He discovers the trail of objects that the animals left, and picks up each one, planning to return them to whoever owns them. He ends up falling into the pit that was originally meant for him and waits for someone to arrive and help him out. He adds, “I sure hope that fellow will be back soon.”