WARNING: This theory may contain spoilers to both Mary Poppins and Chitty Chitty Bang Bang. Read at your own accord.
WARNING 2: This theory may give you a new perspective on both films.
Once upon a time, there lived a wacky inventor by the name of Caractacus Potts, who lived in the beautiful English countryside with his wife, Truly Scrumptious, twin children Jeremy and Jemima Potts, dog Edison, father, former army officer Bungie Potts and the family's fine four-fendered friend, Chitty Chitty Bang Bang. However, this was not always the life Caractacus was known for. In fact, he and his father had initially lived in the charming city of London. You see, Caractacus was also a jack of all trades growing up, going from job to job to help support himself and his father. He even went by the name of "Bert" because it was a nickname that he enjoyed being referred to as since he was a little boy.
The reason for his ridiculous Cockney accent during his life growing up in London was because while his father was British, his mother was originally from America. So, to keep himself from sticking out like a sore thumb, Bert changed his voice to sound more British. While some found his fake accent rather charming, others just simply rolled their eyes at how ridiculous they thought it sounded.
One day, the young Bert had met a beautiful girl by the name of Mary Poppins. From the day they met, the two went on magical adventures or so Bert had assumed. Growing up, he was sure that he dreamt their adventures up because Mary has supposedly assumed that it was all his imagination doing the work. However, one day on an outing with the beautiful girl, Bert came to learn that Mary Poppins truly was gifted in the art of magic, confirming that their adventures were not the product of silly daydreams.
Every so often, Mary Poppins would pop into London to not only check in with Bert, but also to help mend family bonds and cared deeply for the children she took after. In fact, one pair of children she came to love quite dearly were the Banks children, Jane and Michael. During his time knowing the Banks family, Bert also came to love and care for the children and sometimes acted as a surrogate father to the two when George and Winifred
Banks were too busy with their own affairs. One day, Mary Poppins had to leave London when the wind changed. Despite claiming that "practically perfect people never permit sentiment to muddle their thinking," Mary truly did care deeply for Jane and Michael and regretted leaving their lives, even if she had helped in fixing their familial bond with their parents.
Bert eagerly awaited Mary Poppins's return ever since she left after taking after the Banks family. However, as the years passed, Bert came to forget about Mary and moved to the English countryside with his father, Bungie, and went back to going by his old name, Caractacus Potts, dropped his Cockney accent and decided to pursue hiss dream of being an inventor.
During his time living in the English countryside, Caractacus met a beautiful young woman whom he married and had twins, Jeremy and Jemima, with. Unfortunately, when the twins were young, the mother became terminally ill and passed away, much to Caractacus's dismay. Despite all the hardships, the young man managed to raise his two children with his father's help and simultaneously chase his dream of inventing. Until his life would take a turn and he would gain a blast from the past.
After a day of playing in the junkyard, Jeremy and Jemima were driven home by a kind and beautiful young woman, a candy tycoon heiress, by the name of Truly Scrumptious. When he met her, Caractacus had a feeling that she had a familiar aura about her, but he couldn't put a finger on what she reminded him of.
Eventually, when he gained enough money from various odd jobs, such as taking part inn a dance routine at the fair, Caractacus bought a broken down former race car and refurbished it into a beautiful family car. As he revamped the car to be as good as new, he had absent-mindedly fitted the car, which would soon be known by the name of Chitty Chitty Bang Bang, with a pair of beautiful wings, a beak, tail, propeller and raft. One would believe he did so out of the fact that he dreamed of building the ultimate car that could not only travel by land, but also by sea and air.
After unveiling the car, Caractacus took Jeremy and Jemima on a drive to the beach for a picnic. During the trip, due to the sounds the car made, they ultimately decided to name her Chitty Chitty Bang Bang. Along the way, they met up with Truly and invited her along, to which she agreed.
On the picnic, Caractacus had begun telling his children and Truly a story of a brilliant inventor, his family and a magical car named Chitty Chitty Bang Bang. At times, he was not sure if his story was just a mere fantasy he was making up or if it was becoming real the more he narrated. It was during his storytelling that he was beginning to believe that the unfolding story and their adventurous daydream was becoming a reality, but by the time he finished the story, he was brought back to reality and ultimately decided it wasn't real.
After returning home to learn from Truly's father that one his inventions, candy flutes called Toot Sweets, was a success and renamed "Woof Sweets" because they were a hit with canines, Caractacus immediately drove off to tell Truly and ask her to marry him. When driving home to tell his children and father, along with Truly's father, the big news, he came to realize that Chitty Chitty Bang Bang was taking flight. Ever since his marriage,
Caractacus had felt great joy ever since he was a young lad.
During his marriage to Truly, Caractacus came to realize something about Truly. She had kept with her a black umbrella with a handle in the shape of a parrot head, a carpet bag that had infinite space that never ran out, an array of beautiful outfits, coats and hats and a very magical presence about her. Why else would Truly entertain the thought of Caractacus's adventurous story? Why else would she come to love him, along with Jeremy and Jemima the way she would two children he might have loved and known himself? Why would it be difficult to tell his adventurous story from fantasy and reality and how did the car, Chitty, manage to perform magical feats such as flight when Truly was around?
It suddenly hit Caractacus like a ton of bricks and all his memories he had forgotten had come back to him in that instant as he came to only one conclusion. Truly was the same magical woman he had met and come to love since they met during his youth, Mary Poppins! Mary Poppins had helped make Chitty Chitty Bang Bang's magic real and improved his and his family's lives for the better, just like she did with the Banks all those years ago!
Some of you probably would not realize it, but when the crew of Chitty Chitty Bang Bang was casting for the film, they had initially planned for Julie Andrews, known at the time for her role as Mary Poppins, to play the part of Truly Scrumptious. Because she felt the character of Truly was too similar to Mary Poppins, Julie had turned down the role. But think about it, if Julie hadn't said no to the role, in retrospect, "Bert" would have ended up marrying "Mary Poppins." And if you think about that and the fact that some of the crew (songwriter brothers Richard and Robert Sherman, musical conductor Irwin Kostal and choreographers Marc Breaux and Dee Dee Wood) working on the film version of Chitty Chitty Bang Bang was the same crew behind the production of Mary Poppins, the truth in that Albert Broccoli intended Chitty Chitty Bang Bang to be a sequel to Mary Poppins might not be too far off...