On one hand, Jane can be kind, friendly, and caring. On the other hand, it is implied that she, along with her brother, Michael, can be very mischievous. By the time of the film's opening, they had gone through six nannies in the past four months. They try to please their father (whom they address as simply "Father" throughout the film) but have often failed, leading to some insecurity.
At the beginning of the film, the current nanny, Katie Nanna, had quit due to the children's rampant misbehavior. They had gone missing, having run away from her for the fourth time that week. Later, Constable Jones brings them home, revealing that they had not run away, but had merely lost track of her while having trouble with their kite. Jane notes that it wasn't very good, as they had made it themselves. They are then sent upstairs.
Later, Jane and Michael approach their father with an advertisement for a new nanny to replace Katie Nanna. It calls for a fun, caring one. It also hints at the pranks they had played on former ones. However, he dismisses it as ridiculous, and after tearing it up, he throws the pieces in the fireplace. The next morning, Jane and Michael are watching the line of nanny applicants, whom appear to be the exact opposite of the kind they wanted. However, they watch in amazement as a sudden wind blows them all away. Then, a person who fits their advertisement comes to their home.
Later, the woman meets Jane and Michael and amazes them with her bottomless carpet bag. She then measures them with her tape measure. Instead of numbers, it reveals something about the person. For Michael, it says he is "extremely stubborn and suspicious." For Jane, it says that she is "rather inclined to giggle and doesn't put things away." She then allows herself to be measured, and reveals her name is Mary Poppins, which Jane notes as being lovely.
Next, Mary plays a game with them, which turns out to be tidying up the nursery with the use of magic. Jane is able to snap her fingers and toys put away themselves. Afterwards, she and Michael accompany Mary on an outing in the park. On the way, they meet her friend, Bert. He laughs when they say they are going to the park. He is able to trick Mary into transporting them into one of his pictures, which depicts the English countryside. Jane and Michael play at a fair for a time, while Mary and Bert take a stroll.
Later, they meet at Mary's private carousel. Jane is amazed at the idea of that and is even more so when the horses jump off. She accompanies the rest of the group on a ride. She later witnesses Mary effortlessly win a horse race and learns about her special word, Supercalifragilisticexpialidocious. However, the outing is cut short when it begins to rain, which washes the drawing off. Back home, Jane is forced to take medicine to prevent a cold and is surprised when it changes flavor and becomes a lime cordial for her. She and Michael try to discuss the day's events with Mary. When they are about to fall asleep, she sternly denies it.
The next day, Jane and Michael accompany Mary to see about her Uncle Albert, who has floated into the air due to too much laughing. Bert is there as well and is eventually stricken with the "disease" as are Jane and Michael. They have a good time, even having tea while floating thanks to Mary's magic. However, everyone sinks to the ground due to sadness at having to leave to go home. Later, Jane and Michael tell their father of their adventures, which worries and angers him. He sends them up to the nursery as usual, and then decides to try to fire Mary, but is prevented by her planning an idea into his head to take them on an outing to the bank he works at.
The next morning, Jane and Michael go with their father to the bank as "proposed". They are confused and frightened by the descriptions of finance, and simply want to use their money to feed birds at St. Paul's Cathedral. When Michael's is taken by the owner, Mr. Dawes Sr., he and Jane unknowingly cause chaos. Frightened, they escape and eventually run into Bert. They tell him that they believe their father hates them. He helps them to understand that George is simply trapped in a cage due to his responsibilities and has no one to turn to but himself, hence his strict and uncaring attitude toward his family.
At home, Bert is employed by Mrs. Banks to watch Jane and Michael as it is Mary's day off. They assist him in cleaning the chimney but get drawn up it, just as Mary warns them of the danger. She and Bert come after them and decide to lead an exploration of the rooftops. They meet Bert's chimney sweep friends and have a party, which ends up in the Banks' living room. After Bert and the sweeps leave, it is revealed that George is due to be fired because of the events at the bank. To try to fix things, Jane and Michael give him the tuppence that caused the incident.
The next morning, Jane and Michael are saddened as Mary is preparing to leave. They are called by their father, though they think it's not him initially due to the voice being so happy. Going downstairs, they find that he has had a change of heart, and has fixed their broken kite. He takes his family kite flying, symbolizing his intent to become a more involved father and husband.
- An adult Jane Banks would be the narrator for an edition of the Read-Along version, as that role would be reprised by an adult Karen Dotrice.
- In the Mary Poppins novels, she is the eldest of five children, herself, Michael, twins named John and Barbara, and a baby named Anabel, who is born during the second novel.