The Banks family lives in London at number 17 Cherry Tree Lane. They hire Mary Poppins to care for their four small children, not expecting that everything Mary does will have a magical flair. The story progresses in a series of mostly unconnected vignettes about Mary Poppins’ daily adventures with Jane, Michael, John and Barbara Banks.
On Mary Poppins’ first day off of work, she meets her friend Bert the Match-Man. Bert is drawing colored-chalk pictures on the pavement to earn enough money to take Mary Poppins out to tea, but he hasn’t earned a sufficient amount. So Mary Poppins magically transports both of them into the chalk pavement picture, where they eat at a fantasy-world café and ride on a carousel.
Mary Poppins takes Jane and Michael to see her uncle, Mr. Wigg. Mr. Wigg has the unusual ability to float when he laughs, so Mary and the children join him in the air and have afternoon tea in midair near the ceiling of Mr. Wigg’s house.
Miss Lark is an elderly woman who lives next door to the Banks family. She spends most of her time spoiling her small dog, Andrew. The dog does not enjoy the extra attention and wishes to live like a normal dog. Andrew’s best friend is a poorly behaved mutt whose rough outward appearance horrifies Miss Lark. One day, Andrew decides that he will no longer live with Miss Lark unless his friend can live with them. Since Mary Poppins understands animals, she translates Andrew’s demands to Miss Lark, who reluctantly agrees.
One morning, Michael wakes up in a bad mood and decides to misbehave the whole day. He knocks a jug of water out of the housemaid’s hand, kicks the cook in the shin and spills ink over his father’s desk. The worse he behaves, the more he revels in his own bad behavior and looks for opportunities to do more mean things.
Mary Poppins takes all four children for a walk to the park and tells Michael to pick up a shiny object they discover on the ground. The shiny object is a magic compass, which Mary uses to take the children on a brief trip around the world. They visit all four corners of the globe in a few minutes, pausing in each location just long enough for Mary to have a conversation with the local animals.
Michael is still in a bad mood and demands that Mary give him the compass. When she refuses, he waits for Mary to fall asleep later that night and takes it. When he tries to travel around the world, he suddenly sees all the talking animals rushing toward him as if to attack him. When he yells for Mary to save him, the animals vanish. Mary scolds Michael and says he shouldn’t have touched her property. She brings him a cup of warm milk. He goes to sleep, his bad mood finally at an end.
Mary and the children go to a gingerbread shop that Jane and Michael have never seen before. They meet the cheerful old shop owner, Mrs. Corry, and her two grown daughters, Fannie and Annie, who are so tall they are almost giants. Jane and Michael receive paper stars in the wrapping of their gingerbread and tell Mrs. Corry that they always save paper stars.
Later that night, the children wake up and see Mary Poppins removing their paper stars and walking out past the front gate to meet Mrs. Corry, Annie and Fannie. The four women proceed to climb ladders and paste the paper stars onto the sky with glue. The paper stars then turn into real stars.
Jane and Michael go to a party, leaving Mary Poppins alone with the twins, John and Barbara, who are still babies. Because of their innocence, John and Barbara can talk to animals and to elements of nature, such as the sunbeam that shines into their nursery. John and Barbara discuss how strange it is that Jane and Michael never seem to understand the language of the wind or the tweets of birds.
Mary Poppins explains that they once did. She says that all children stop understanding the language of nature after they turn 1 year old. John and Barbara cry and insist that they won’t forget. A starling bird, which is friends with the twins, returns to visit after their first birthday but discovers that they no longer recognize it and can’t communicate with it.
Michael wonders what happens at the zoo at night, so Mary Poppins takes him and Jane to the zoo at nighttime, where a bear in a guard’s uniform admits them. The children walk around with all the animals, which are walking outside their cages, observing various humans inside the cages. They learn that this kind of topsy-turvy night is caused by magic that only occurs when Mary Poppins’ birthday falls on a full moon.
Mary Poppins talks with a hamadryad snake (king cobra), which gives her his shed skin as a birthday present. All the animals in the zoo form a circle and dance around Mary Poppins in celebration. The children find themselves falling asleep, and they wake up in their own nursery. Mary Poppins insists she never went to the zoo the previous night, and the children assume they must have been dreaming until they see that Mary is wearing a snakeskin belt, her birthday present from the snake.
Mary Poppins and the children go Christmas shopping. As they are about to leave the store with their purchases, they meet a very excited child who appears to be wearing a dress made out of a piece of the sky. Her name is Maia, and she is one of the stars in the Pleiades star cluster.
Maia has come to earth to buy Christmas presents for the rest of her star sisters. She wants Jane and Michael to help her select presents. Maia is about to return to the sky without a Christmas present for herself, but Mary stops to give Maia her own highly-prized fur gloves.
On the first day of spring, Mr. Banks announces that the wind is blowing in from the west. Jane and Michael hear this news with despair, because Mary Poppins once told them that she would only stay with their family until the wind changed directions. Mary is uncharacteristically pleasant all day, and when she goes downstairs after serving the children their supper, she doesn’t return.
Jane and Michael run to the window and see Mary on the ground with her traveling bag in her hand. As they watch, Mary opens her umbrella, and the west wind blows her away. They feel encouraged when they discover a letter left by Mary, saying that she will return someday.