The movie starts with a man named Jack riding around the city as he turns off all the lamps. He seems to be enjoying his job pretty well, as he also sings loudly and interacts joyfully with passersby. Not long after, we meet the Banks family, who happen to be having a rough morning after the housekeeper, Ellen, reports a burst pipe. As Michael, the head of the family, moves quickly to help Ellen clean up, his three children, Annabel, John, and Georgie, also offer their help, with Annabel calling a plumber.
Michael’s sister, Jane, is also at the house as she came to greet the family. Amid the chaos, Ellen hears bangs on the door, and when she goes to check, thinking it’s the plumber, she sees two men, Mr. Gooding and Mr. Frye, placing a notice on the door. The men say they’re lawyers, and they need to see the owner of the house, but Ellen has too much to think about inside the house, so she slams the door back.
As the knocking continues, Michael welcomes the lawyers into the house while the others take over with the cleaning. Since Jane is around Michael when the men are about to discuss with him, she’s mistaken for Michael’s wife. However, Michael clears that up and says his wife died about a year ago, and the men share their condolences. Gooding and Frye then proceed to introduce themselves properly as solicitors for a law firm who are there to leave Michael with a few days’ notice to pay up the loan he took from the bank.
Jane is shocked to hear this, and Michael confesses to taking the loan when things were quite difficult for him. He also says that his wife used to be in charge of the family’s finances, and with her gone, he has been having a hard time keeping up with it. As he’s about to offer the men some money as part of the loan payment, they tell him the terms have changed, and Michael will now have to pay the entire money in full before the end of the week. They also tell him that if he doesn’t meet up, then he’d have to vacate the house because the bank would be repossessing it.
This is all too much to take in for Michael, who doesn’t earn enough to pay the money immediately, even though he works for the same bank he borrowed from. At that point, Jane remembers that their father had shares in the bank, and they could use it to save the house. Despite being reluctant at first to the idea, Michael asks Gooding and Frye if he can use the shares, and the men tell him to bring the share certificate to the bank in order to claim and use it to pay what he owes.
When the men leave, Jane comforts Michael, who is terrified as he didn’t think the loan situation would result in him losing his house. Jane offers to help him check around the house for the certificate, and as they’re talking, the children hear the conversation. Michael and Jane immediately assure the kids that they’re not going to lose the house. Afterward, Michael gives them some money to go and get groceries in the market.
While the kids are away, Jane and Michael start checking around the house, with Michael starting from the attic. Seeing as there are a lot of his wife’s belongings still there, Michael becomes emotional and wishes his wife was still around as he knew she’d always come up with a solution. Jane soon enters the attic after an unsuccessful attempt at finding the certificate in their father’s old wardrobe. Jane’s attention is suddenly drawn to a kite they used to play with as kids, but there seems to be too much on Michael’s mind that he doesn’t seem interested and even decides to trash the kite.
After he does, a strange wind blows, and it swings the kite out of the trash, after which it continues to fly around the city.
Meanwhile, the kids are still on their way to get groceries when Georgie sees the kite and chases after it. Annabel and John try to get him, but a gardener stops them from taking Georgie’s route. Georgie gets a hold of the kite, and as he tries to control it, a strange wind blows again to the extent that it lifts Georgie off the floor. While riding away from the storm on his bicycle, Jack sees Georgie and immediately stops to save him. While they both then try to drag the kite down, the storm suddenly clears, and a figure appears to be bringing down the kite from the sky.
While Jack recognizes the figure as Mary Poppins, Georgie is left intrigued by how she’s flying down with the kite. As Mary Poppins lands, she scolds Georgie for almost losing the kite and also complains about Annabel and John looking dirty as they arrive at the scene. Jack compliments her, and she thanks him before saying she’s going to the Banks family house to get a job as a nanny, seeing as the kids could use one. Annabel and John don’t seem to agree, but little Georgie follows her as he’s already blown away by what he saw.
When Mary Poppins arrives at the house, Jane and Michael are shocked to see her, and they tell the kids that Mary Poppins used to be their nanny when they were kids. Just then, Georgie mentions what he saw Mary Poppins do with the kite, but Jane and Michael brush him off, saying it’s impossible, and as kids, they also used to believe she could do things like that even though it wasn’t real. As Georgie tries to explain further, he hears Mary Poppins’ stick talk. When he tells the others, they believe he’s only under an illusion.
The kids go to their rooms, and Mary Poppins tells Michael and Jane that she’s there to take care of the kids as she once did with them. While Michael doesn’t think it’s a good idea since he still has a loan to pay off, Jane happily accepts her offer, with Michael not too happy with this. Georgie makes sure to watch Mary Poppins as she comes towards his room. After watching yet another trick, he asks her how she did it and why others believe it’s impossible. She brushes him off the subject, but her stick talks again, saying adults tend to forget when they’re older.
Mary Poppins finally gets to arrange her things. After answering several questions from the kids, especially Annabel and John, who believe they’re old enough to be without a nanny, she tells them to head over to the bathroom for a bath. There, the kids lay down some rules about how they like their bath, but Mary Poppins doesn’t look bothered as she prepares the water and also throws a few things into it. After she’s done, Mary Poppins asks the kids to get into the bathtub, and as Georgie does, he suddenly disappears inside the water. John and Annabel also go in, after which Mary Poppins follows. It suddenly appears like they’re deep inside the ocean, but this is not a concern for the kids, who seem to be enjoying their magical bath. After a long time inside, Mary Poppins gets the kids out, and the three of them look delighted.
Elsewhere, Jane and Michael are still looking for the share certificate without success. Michael suddenly gets frustrated when the kids run over to him, talking about their bath. He, however, regrets it and apologizes to them. As Jane is also trying to let the kids know that their father didn’t mean to shout at them, she remembers that she and Michael’s father had a safety deposit box in the bank. Since the certificate could be there, Michael and Jane agree to go and check it in the morning.
The next morning, Jane and Michael head over to check the safety deposit box, but they’re again unlucky as they don’t find the certificate there. As Michael is lamenting, Jane asks if the Bank Manager, Mr. Dawes, couldn’t give him an extension on when to pay the loan. Michael says Mr. Dawes is no longer at the helm, and his nephew, Mr. Wilkins, is now in control. Seeing as there’s no harm in still seeing the new manager, Jane tells Michael she’s headed to see Mr. Wilkins. Even though he doesn’t want this, there’s nothing he can do as Jane is faster than him. When they get to the office, Mr. Wilkins looks friendly and welcomes them in. They narrate their ordeal to Wilkins, and he tells them there’s nothing he can do about an extension but offers to help them check the bank records. After he checks the record books in their presence, Wilkins tells Jane and Michael he didn’t find anything. Still, he assures them that he’ll keep searching through all the bank records to ensure they find the certificate and meet the deadline.
With this, Michael and Jane leave with some hope, but it turns out Wilkins isn’t as much of a charmer as he looks. He makes a call to his secretary requesting all the files belonging to Michael’s father, and right there, he tears a page with proof of their shares and throws it into the fireplace in his office.
Back at the house, John suddenly thinks of an idea to save their house while helping Ellen with some chores. He runs over to tell Annabel they can sell their mother’s special bowl, which she always called priceless, to make some money to help their father and save the house. While both of them are talking about John’s plan, Mary Poppins is also talking to Jack just by the balcony.
Meanwhile, Annabel disagrees with John’s plan, and Georgie, who doesn’t even know what they’re talking about, starts dragging the bowl with them, asking that it be put back in its place. As a result, the bowl falls, and a small part of it breaks off. They immediately try to get the broken piece before Mary Poppins finds out, but it’s too late as she shows up with the piece, asking who broke it. While the kids are shifting the blame on each other, they hear a voice saying it was the three of them. Much to their shock, the voice came from a drawing on the bowl. The kids also realize that the drawing on the bowl has changed, with an initially perfect carriage now having one of its wheels broken.
Mary Poppins offers to help the character from the bowl to fix the wheel, and just as the children wonder what she means, she spins the bowl, and that magically takes them inside the drawing along with Jack. Mary Poppins doesn’t waste any time in fixing the wheel with her scarf, and just as she suggests going back, the kids say they want to stay back and enjoy a carriage ride. Jack also says he’s interested, and that convinces Mary Poppins to allow them to stay back and ride the carriage. To make the adventure more interesting, Mary Poppins says they’ll be headed to a music hall, much to the excitement of the kids.
They’re warmly received when they get to the hall, especially because of Mary Poppins’ presence. Jack then goes on to host the show, with Mary Poppins also singing to entertain the kids and the audience. Just as Mary Poppins ends her electric performance, Georgie goes outside and sees some bandits leaving with items from their house. Georgie gets furious and asks the bandits to return all they took, but the leader of the group tells him the items are no longer for them because they’ve lost their house.
As John and Annabel see what is happening, the bandits immediately leave, kidnapping Georgie in the process. Annabel and John soon get help from the carriage that brought them to the music hall, and with that, they’re able to save Georgie. However, the bandits’ leader is not going down without a fight. He breaks part of the road the carriage is passing through, and that lets the kids fall continuously into a black hole. This seems to send the kids back into the real world, though the events inside the bowl turn out to only be a nightmare as they’re all in bed already. What they don’t understand, however, is how they all had the same nightmare.
The kids are also worried about losing their house, and to get them to sleep, Mary Poppins tells them that she has someone who can help them fix the broken piece of the bowl they plan to sell in order to save their house. The next morning, Michael forgets his briefcase, and Mary Poppins uses this as an excuse to take the kids out to the repair shop. As they get outside, Jack happens to be there and decides to give them a lift on his bicycle in an improvised way.
Not long after, they reach the shop of a lady named Topsy, who happens to also be Mary Poppins’ cousin. Despite being reluctant to answer them because she didn’t want to work on that day, Topsy finally lets them in and agrees to fix the bowl. However, the kids have their hearts broken when Topsy tells them the bowl is not worth very much. Seeing as it’s not going to be enough to save their house, the kids look very sad, but Mary Poppins and Topsy try to cheer them up.
After leaving Topsy’s repair shop, they head over to the bank to return their father’s briefcase. While Mary Poppins is asking the desk officer to call Michael and inform him that they’re around, the kids see the lawyers who came around to their house the other day. They then decide to follow the men sneakily to see if they can convince them not to take their house away. Even though Mary Poppins sees them leave, she doesn’t seem to have a problem with it. As the kids reach Wilkins’ office, which was where the lawyers entered, they start hearing Wilkins as he tells the lawyers not to give Michael an extension. Even though Frye tries to make a case for an extension because Michael works for the bank and has had a tough few months, Wilkins is still determined to get the house.
Seeing how Wilkins handles his pocket watch, the kids liken him to the leader of the bandits, and Georgie barges in, calling Wilkins a thief and asking him to leave their house alone. Wilkins is informed that they’re Michael’s kids, and just as he tells the lawyers to get them, the kids run back downstairs. Coincidentally, Michael has just received his briefcase from Mary Poppins, and as he asks for the kids, he sees them running down the stairs and causing a scene as they call Wilkins a thief. Wilkins doesn’t take any time to reach the scene to defend himself and also offer his fake support to Michael regarding the house. He also seems to threaten Michael that his kids’ behavior could’ve cost him his job.
For this reason, Michael is furious with the kids and asks Mary Poppins to take them home immediately. On their way home, they get lost in the fog, and as they try to figure out their way home, they see Jack, who offers to cheer them up. Together with his fellow lamplighters, Jack sings and entertains the kids.
Time goes so fast, and by the time the kids get home, Michael is already there. He shouts at them again because they didn’t obey his instructions. He also lashes out at Mary Poppins for not bringing the kids home in time. While inside the house, Michael continues to blame the kids for almost making him lose his job. As he talks, he suddenly breaks down into tears, lamenting that he doesn’t know what to do because it seems he has run out of options to save the house. The kids are very thoughtful with their next move as they cheer their father and say they were only trying to help.
Days pass without any changes to Michael’s fortunes as the final day arrives, and he and the kids pack their belongings into a tow truck. As they prepare to leave, Michael’s neighbors come over to say their goodbyes. While they’re at it, Georgie comes out with the kite as the last thing he almost forgot while packing. Michael is surprised to see the kite again, and as he takes a closer look, he realizes that the kite had been patched up with the share certificate they’ve been looking for all this while.
With only seven minutes left until the bank’s deadline at midnight, it looks unlikely that Michael will be able to get there in time to show Mr. Wilkins the certificate. However, Mary Poppins says it’s possible, and he should get going along with Jane while they leave the rest to her. After Jane and Michael leave, Mary Poppins asks Jack to get his other lamplighters as they’ll now go ahead to slow down the time on the town’s big clock. Jack does a wonderful job, and with the help of Mary Poppins, their mission looks accomplished.
Elsewhere at the bank, Wilkins looks fulfilled as he’s sure that Michael will be unable to save the house with the little time left. However, as his own clock hits midnight, he’s surprised that the town’s clock doesn’t ring, even though he’s sure his time is correct. Just as he sees that the town’s clock still has some minutes before midnight, Michael and Jane arrive at the bank. Wilkins is still bent on taking the house, and he tells Gooding and Frye to lock the doors to prevent them from entering.
Seeing that they made it in time, Michael and Jane try to immediately go into the bank, but they see that the doors have been locked. They then try to call Wilkins from his window, but he ignores them. Michael eventually devises a plan to get the kite into Wilkins’ office, and just as they try the door again, Frye, who has always had a soft spot for Michael and his family, pushes Gooding away and opens the door.
Michael’s kids, Mary Poppins, and Jack soon join Michael and Jane at Wilkins’ office, where they’re trying to assemble the parts of the certificate. Even though most of the pieces are there, the part with the signature is missing, and that gives Wilkins leverage to reject their attempts to save the house. At that point, Michael realizes that Wilkins had been against him all along, and it was now clear since he was refusing to consider the certificate even though he knew it was proof. Michael is done trying to save the house, and he tells his kids they’ve got to leave as he’s sure they’ll be fine without the house.
As he’s about to head out with his family, they all hear a voice, and it turns out to be Mr. Dawes, who is now back to take his position as manager. He accuses Wilkins of trying to cheat Michael and then asks Gooding and Frye to lead him out. Out of anger, Wilkins lashes out at his uncle before leaving.
Mr. Dawes is happy to be back, and he tells Michael his father’s shares in the bank are still very valid, and he can also keep the house.
Days later, at the town’s spring fair, the family enjoy themselves and even get balloons that magically lift them into the sky. After the fair, the family heads home, and just as they open the gates, they are met with a strange wind. This appears to be Mary Poppins’ cue to return to where she came from, and she does just that in the same manner she came, except this time she uses her umbrella and not a kite.
As soon as Michael’s kids run into the house, Michael and Jane are sure that the wind means that Mary Poppins is now gone. To appreciate her efforts for his family, Michael faces the sky to say goodbye and thank you to Mary Poppins.