The film begins during the Second World War in 1941 with AA Milne (Domhnall Gleeson) nicknamed “Blue” by his friends and family, who receives a disturbing telegram at home where he lives with his wife Daphne (Margot Robbie). After that, the events go a few decades earlier when Blue is in World War I fighting in the Battle of Somme, and then resumes its life in England but suffers from post-traumatic stress.
Milne and Daphne then conceived a child, and even if Daphne wanted her baby to be a girl, she gave birth to a boy whom Christopher Robin baptized, but he relished Billy. They hire a lady for her named Olive (Kelly Macdonald) who is given the primary responsibility for raising children. Blue faces difficulties in resuming his writings – he is trying to make a powerful message against the war – and moves with his family to a country house.
Daphne does not agree with this move, and even goes back to London for a longer period. When Olive is forced to leave to take care of her dying mother, Blue and Billy (Will Tilston) are left alone for a while. Initially hesitant, Blue takes Billy to a walk through the woods and begins to invent stories about the boy’s adventures with his plush animals that his parents have bought over time. Blue invites his illustrious friend Ernest to visit them and together they set up the children’s books Winnie-the-Pooh.