"In every job that must be done, there is an element of fun. You find the fun and the job's a game."
In 1910, London, at 17 Cherry Tree Lane, the Banks family finds themselves in need of a nanny. Katie Nana finally had her fill of the mischievous Banks children, Jane and Michael, and their antics and proceeds to storm out once Mrs. Banks comes home. Mr. Banks is a busy businessman, a partner in a local bank, and Mrs. Banks is a political busybody who spends more time out at her rallies for women's votes than at home. Jane and Michael draw up a list of desirable traits they want in their next nanny, but Mr. Banks tears it up and tosses the pieces of paper into the unlit fireplace.
The following day a long line of potential nannies is standing outside of the Banks household, much to Jane and Michael's dismay none of them seem to match their ad. Just before the first applicant is shown in, a large gust of wind literally blows all the nannies away and Mary Poppins arrives via umbrella. Upon entering, she insists that she never gives references and proceeds to read from the torn up paper that Mr. Banks had tossed aside. Mr. Banks is so flustered by this that Mary easily talks her way into the job and proceeds upstairs to the nursery without so much as giving Mr. Banks her name.
Jane and Michael are delighted by their new nanny's seemingly magical feats, even if Mary herself never acknowledges them. Her first task for the children, cleaning up the nursery, becomes a game with a snap of her fingers and afterwards she takes them on their promised outing. Although, it wasn't to the park as they expected. Goaded into using her magic by Bert, all four of them jump into a chalk drawing of the English countryside, dressed for the occasion. Mary and Bert enjoy some time together as the children explore the merry-go-round, they also participate in a horse race (which Mary wins by politely asking her way to the front of the pack), and sing a silly song before rain interrupts the rest of their plans.
Later that evening, Jane and Michael are very excited about their day, asking Mary how long she intends on staying, and recounting their fantastical outing. Mary flatly denies winning the horse race and using any sort of magic, lulling the children to sleep with a song whose words encourage the listener to stay awake.
The next day, Mary takes the children on a set of errands but is quickly interrupted by Andrew the dog, who tells Mary that Uncle Albert is on the ceiling again. So, she hurries over even with the children in tow, meeting Bert there too. Eventually everyone except Mary is affected by Uncle Albert's laughter, until Mary deems it is time to return home. The sad thought forces the group back to the ground.
Mr. Banks calls Mary aside, flustered and annoyed that his ordered life is seemingly turned upside down with his children's flights of fancy (which he firmly blames on Mary). During the course of the conversation she convinces him to take the children to his workplace the next day, to see what adulthood has in store for them. On the way to the bank, they encounter the Bird Woman, whom Mary had spoken of (well, sung of) the night before. Michael wanted to spend his tuppence for a bag of bird seeds, but his father admonishes the idea and calls it a waste. At the bank, Mr. Banks' employeers almost succeed in taking Michael's tuppence from him to open an account, but when Michael protests loudly this causes a run on the bank. This was something that hadn't happened since the 1770s, and likely Mr. Banks would be facing harsh consequences for it. The children flee in the ensuing chaos, through the darker parts of London, until they run into a familiar face.
After the children are returned from their harrowing adventure to their home by Bert, he proceeds to start to clean the chimney. Mary Poppins returns from her day off, admonishing Bert that he shouldn't let the children by that close, right as Michael is sucked up the floo. Jane follows after, and Mary being Mary follows after with Bert in tow. After powering her nose with more soot, she marches the children and Bert across the roof tops of London, up a smoke staircase, to admire the view. They are later joined by other chimney sweeps, eventually returning back to the Banks residence just as Mr. Banks arrives home. The man is highly agitated by this, demanding Mary explain herself, to which she very solemnly replies that she never explains anything and whisks the children upstairs. Mr. Banks receives a phone call to return to the bank for disciplinary actions, and Michael gives his father the tuppence, hoping to make things right again.
The next morning, the wind has changed, heralding Mary's departure. The children are very sad, begging her to stay. Their father has gone missing but suddenly he bursts from the basement with Michael's mended kite and the entire family goes out to the park. Mary watches them leave, happy that things had been set right, but perhaps a little wistful as well. The Banks family does not bid her farewell, rather Bert, who asks her to not stay away so long this time.
Mary Poppins is practically perfect in every way. The end.
On a more serious note, however, Mary has a few different sides to her personality. She is the ultimate professional, stern and aloof in most of her dealings, especially with most adults. Sometimes she shows this side to the children as well, but more often than not she has a professional front, so to speak. This is especially apparent at the end of the film as she's packing up to leave, the children begging her to stay. After they leave to go spend time together as a family, Mary has a brief conversation with her umbrella, who points out they seem to be ungrateful for what she's done. Mary replies that it was "as it should be" and "practically perfect people never permit sentiment to muddle their thinking". She remains aloof because getting attached is simply something she can't do. There are other children and families to mend, the wind has changed, and so Mary must be on her way. She never stays in one place for very long, and as such, tries to avoid too many personal entanglements. Bert seems to be one of very few exceptions to this.
Mary has a stern side that frequently comes to light, whether dealing with the children, the adults, or the adults that act like children. Even concerning her magic, she's very no-nonsense and often there's no room for protest. The children pick up on this pretty quickly as the very first thing they do after Mary is hired is clean the nursery. She has rules and expectations of her charges, and adheres to them before anything else. However, that's not to say that Mary is a cold or distant person.
Mary also has quite the dry sense of humor, even coming across as rather sassy at times, as such when her reflection continues singing in a rather show-offy kind of way. Mary gives her reflection quite the look and remarks that her reflection is cheeky. When Bert pretends to walk a tight rope walk for Jane and Michael, she literally does a slow clap while trying not to roll her eyes (knowing that Bert is just trying to get her to use her magic, after all!).
In nearly every circumstance, Mary seems rather unflappable. In the books it's suggested that she's traveled extensively, even to magical dimensions and "exists in every genre at once", meaning she's aware of the supernatural, sci-fi, horror, and so on, even if she doesn't understand it all. While visiting Uncle Albert, everyone is in a fit of hysterics while Mary simply goes about the business of pouring the tea, keeping a straight face all the while. Later, at the end of their adventures on the rooftops of London, Admiral Boom sets off fireworks in their direction. Mary simply stands there as everyone else ducks for cover.
It could also be said that Mary is a little manipulative, but in such a way that most people either hardly notice, or her will is simply so much that they don't think to argue until after the fact. She pulls this the most on Mr. Banks, basically hiring herself as his nanny. She also convinced him for a time that taking Jane and Michael to the bank was his idea. She is also slightly vain, at various points in the movie she's either making sure her hair is still in place or merely looking briefly in a mirror. Unlike the books, however, Mary Poppins is not so vain as to ignore her duties or abandon her sense of politeness towards others.
There's a nurturing, caring side to Mary that obviously shines when she's dealing with the children but also to animals and people in general. Even if she is aloof, she's extremely polite, such as when she asked some of the jockeys in the horse race if they would kindly move aside for her. She understands a child's needs for creative outlets, but within distinct boundaries. Often times she leads the children to this conclusion through their own actions, such as the time they refused to sleep so she encouraged them to stay awake by singing to them (A lullaby, of course. She is tricky like that.). She's also very child-like, in her wonder of things happening around her. Mary's absolutely delighted by the antics of the penguins, the barn yard animals, and she can't contain a smile at Bert's exuberance either. Her sense of wonder at the magic in the world, even in ordinary things is something she tends to hide, though around her closest friend Bert, and the children, she does let that slip.
Mary Poppins very comfortably straddles the world of adults and children. She is both mature and wise, playful and childish. She retains a sense of wonder with the world that most children have and most adults do not. Despite a distant, professional appearance, Mary can also be kind and caring towards others. The quintessential British nanny from the turn of the century, with a dash of magic for good measure.
In the books Mary Poppins is called "The Great Exception". That is she never outgrew the magical abilities that all children have. In general her magic seems to be based on that child-like innocence and affinity for the whimsical.
Talking to animals - what is says on the tin, really. She can understand the barks of Andrew just perfectly.
Animating objects - With a snap the toys in the nursery come alive, although it never seems to be anything particularly large.
Manipulating reality - To varying degrees, although I suspect it's limited to only a few people at most. Most notably jumping into a chalk drawing, or creating a staircase out of smoke, or changing the flavors of the medicine. Nothing permanent, either.
Defying gravity - Flying in via umbrella, for instance! Or sliding up and down stair banisters at a controlled speed.
Precognition(?) - She knows the entire Banks family by name before actually meeting them in person.
She also possess a strong will, so bending will come fairly naturally, I would imagine.
In a relationship with Peter Smith (aka Prisoner Number 6).
Currently bothering most of the children in Nautilus.