Decades after her original visit, the magical nanny returns to help the Banks siblings and Michael’s children through a difficult time in their lives.
London is in an economic depression, and Michael Banks is about to lose his childhood home that he now lives in with his three children – John, Annabel, and Georgie. What’s more is that Michael’s wife passed on, leaving him to raise the kids on his own, with occasional help from his sister Jane. This summons magical nanny Mary Poppins back to London to help the family, along with a lamplighter named Jack.
Michael knows that his and Jane’s father kept a certificate of ownership to shares in the bank that would help pay off the loan that can save his house. However, the bank’s greedy president, William Weatherall Wilkins, is planning to take Michael’s house, as well as other people’s houses.
Through Mary’s magic and some help from Jack and his fellow lamplighters, the children are able to find a new perspective on things, as well as accept that their mother is alive in their hearts.
Michael finds the certificate in pieces used to patch up an old kite. He and Jane rush to the bank before the deadline while Mary, Jack, and the lamplighters try to buy them time by literally turning it back. They find that they don’t have all the pieces to the certificate, but thankfully they are saved when Wilkins’ uncle, Mr. Dawes Jr, arrives and backs Michael and his family, and then fires Wilkins. Dawes tells Michael that his father had enough money saved that it’s more than enough to save the house.
After regaining the same sense of childlike wonder he once had, Michael and Jane realize that Mary Poppins must once again leave, but they thank her quietly for all that she did for them.
The film opens in London, during a time that is being called a Great Slump. A lamp lighter named Jack (Lin-Manuel Miranda) rides his bike around town, singing about the city’s beauties despite the depression (“Underneath The) Lovely London Sky”). He rides down Cherry Tree Lane where Admiral Boom (David Warner) complains to Mr. Binnacle (Jim Norton) that Big Ben’s chimes are coming in late.
Michael Banks (Ben Whishaw) still lives in his parents’ home as a widow with his children John (Nathanael Saleh), Annabel (Pixie Davies), and Georgie (Joel Dawson), along with housekeeper Ellen (Julie Walters). Michael’s sister Jane (Emily Mortimer) is there as well, trying to help her brother handle things around the house. Two lawyers from Fidelity Fiduciary Bank, Hamilton Gooding (Jeremy Swift) and Templeton Frye (Kobna Holdbrook-Smith), show up to leave a notice of repossession on the front door. They explain that because Michael has failed to pay back a loan, his house is to be taken by the bank unless he pays it back by Friday at midnight. Michael remembers that his father kept a certificate of ownership to some shares in the bank, and he goes upstairs to find them, but he starts to worry about handling things without his wife Kate around (“A Conversation”). Jane joins him upstairs to help look for the certificate, but they instead find an old kite from their childhood. Michael dismisses it as garbage.
The children go off to the shop, when the kite blows away and catches Georgie’s eye. He runs after it but is pulled away by the wind, and John and Annabel run after him. Georgie is caught by Jack, who helps him fly the kite properly, but it gets pulled in by the wind. The clouds then separate, and to their surprise, down comes Mary Poppins (Emily Blunt) floating with the kite. She addresses the children by their names and greets Jack, knowing him as an apprentice of Bert the chimney sweep. Mary then guides the children back home.
Upon returning to the Banks home, Michael and Jane are stunned to see Mary having come back after all these years. She tells them she is there to look after the Banks children, meaning both Michael and Jane and Michael’s kids. Mary heads up to her room to settle down. Michael and Jane remember Mary but aren’t sure if all the magic they witnessed was real.
Mary has the children take a bath due to them being covered in filth from running around through the park. She adds a magical soap to the water and has the children jump in to be taken to an underwater world (“Can You Imagine That?”), leaving the children in awe.
Michael and Jane go to the bank to speak to its president, William Weatherall Wilkins (Colin Firth), to discuss the matter of George Banks’s shares that could help save the house. Wilkins looks through his documents but claims to find nothing relating to the Banks family. He assures Michael and Jane that he will continue to look, but after they leave, he tears out the page with the Banks name on it and he throws it in the fireplace.
That night, as Mary is talking to Jack, the children discuss selling their mother’s China bowl to save their house. The three have an argument over it, which leads to them dropping and cracking it. Mary picks it up and shows the children that the man drawn on the bowl is talking to them because they damaged his carriage. Mary spins the bowl and transports herself, Jack, and the children into the animated world of the bowl. They meet the coachman Shamus (Chris O’Dowd), who is a dog, and his horse Clyde (Mark Addy). After fixing the carriage, Mary, Jack, and the kids join Shamus on a carriage ride as they head to a new destination (“Royal Doulton Music Hall”).
The group arrives at the music hall where Mary and Jack put on a performance with the other animal characters (“A Cover is Not The Book”). Georgie gets distracted by something outside, and he sees a wolf, a badger, and a fox (all representing Wilkins, Gooding, and Frye) taking the family’s possessions away. They kidnap Georgie, prompting John and Annabel to go save him. The kids chase the villains through the bowl until they reach the edge. The wolf cracks the bowl and causes the children to fly over the edge of the bowl.
Georgie wakes up as he was supposedly having a nightmare. Mary comforts him and his siblings as they miss their mother (“The Place Where Lost Things Go”), assuring them that she lives on in their hearts.
The next day, Mary and Jack take the children to get the bowl fixed. They go to a shop owned by Mary’s eccentric cousin Topsy (Meryl Streep). Mary forgot that they arrived on a second Wednesday, which means things go literally upside down for Topsy. With Mary’s help, Topsy and the children are able to see things from a new perspective (“Turning Turtle”). Topsy agrees to fix the bowl, but she tells the children it’s not worth much, and that their mother calling it “priceless” probably just meant that it was worth much to her.
After leaving the shop, the group runs into Jane on the street as she is helping organize a rally for labor workers. Jack escorts her since he’s had a crush on her since they were children. Mary and the children go to the bank to find Michael, but the children head on up to Wilkins’ office where they overhear him plotting to take not only their house, but the homes of other people in London as well. They burst into his office and threaten to expose him to their father, and they run down to the lobby. When they find Michael, Wilkins denies anything about a plan to take the house, leaving Michael angry at the kids for nearly getting him in trouble.
Mary and Jack walk the children back home as they run into Jack’s lamp-lighting buddies, who help guide them home (“Trip a Little Light Fantastic”) while also showing them how to speak their lingo. When they get home, Michael appears distressed amid his worries for losing the house. The children try to cheer him up and echo Mary’s words to him regarding their mother.
The next night, the family has packed all their belongings and are preparing to head out. The neighbors gather around to say goodbye. Georgie then brings out the kite, and Michael takes a look to see that it’s been patched up with a drawing he made of his wife and kids. When he looks closer, he sees that the drawing was made on the certificate that he had been looking for. They see that they have seven minutes to midnight, which means they won’t make it before the deadline, so Mary, Jack, and the lamplighters gather to turn back time for the Banks family.
Michael and Jane rush to the bank while the lamplighters gather around Big Ben and try to get Jack up to the top to reach turn back the hands. He gets inside and dims the lights so nobody can see what they are doing. Meanwhile, Wilkins is with Gooding and Frye as he waits for midnight to claims the Banks home. Mary floats up to the clock just before it strikes midnight, and she turns it back five minutes. Thinking the time is wrong, Wilkins continues to wait as Michael and Jane try to get his attention with the kite. The wind blows it inside his office, and Frye (being the only good one in the bank) lets them into the building. The children join them as they try to put the pieces together, but they are missing the piece with the signatures, which Georgie threw out. Feeling they have lost, Michael hits back at Wilkins and says he can take the house because he still has his family. Just then, Wilkins’ uncle, Mr. Dawes, Jr. (Dick Van Dyke) comes back from his travels and criticizes his nephew for his dirty deeds against the people of London. Dawes comes back to take over, and he fires Wilkins. Dawes tells Michael that he knew his father and agreed that he made some good investments, so the shares will be enough to pay off the loan, and the family can keep their house.
The following day, the family goes out to the park. They encounter a woman selling balloons (Angela Lansbury), who tells the family to pick the right balloon. Michael admits that he hasn’t held a balloon since he was a child, and the balloon lady tells him that he’s forgotten what it’s like to be a child. He grabs one, and it sends him floating in the air. The children follow suit, along with the other park patrons, plus Jane and Jack. Wilkins attempts to join, but he grabs the wrong balloon and it falls to the floor. Michael and his family float among others as they feel happy and childlike (“Nowhere To Go But Up”).
As the family returns home, a gust of wind opens the front door. Mary senses this and feels it is time to go. Michael and Jane realize this, and they quietly thank Mary for everything that she has done. Jack rides his bike around town again, and Mary floats up to the skies on her umbrella, taking one last look behind her and smiling.