Most of us are familiar with the classic story of Mary Poppins, but what about the nanny's life before she tended to the Banks children? While little is known about her past, it's pretty clear that Mary has been to Cherry Tree Lane before. Not only is she familiar with the town, but she's awfully friendly with the chimney sweep, Bert. Though the state of Mary and Bert's relationship has always been a little fuzzy, a popular theory has recently resurfaced ahead of Mary Poppins Returns explaining their interesting bond: Mary Poppins (played by Julie Andrews in the original and Emily Blunt in the sequel) was actually Bert's nanny when he was a kid.
In the very beginning of the film, Bert sets the story by telling the audience, "Wind's in the East/Mist coming in/like something is brewing/about to begin./Can't put me finger/on what lies in store/But I feel what's to happen all happened before." He could be insinuating that his own story is about to be told through a different set of children. It could explain why Bert knows so much about Mary's magical abilities, because he witnessed them first-hand when he was a boy.
A few other clues that Mary Poppins was Bert's nanny are dropped throughout the original's musical numbers. During "Supercalifragilisticexpialidocious," Bert explains that he first learned the word when he was "just a lad" and his father told him he was bad. Maybe Mary came into his life when he had family struggles like Michael and Jane. Bert's upbringing is further teased in the song "Jolly Holiday" when Mary sings, "Though you're just a diamond/In the rough, Bert/Underneath your blood is blue!" Hmm, so is Bert from a blue blood family? If so, why is he sweeping chimneys for a living? Well, that brings us to the biggest twist of this theory: Bert is actually Mr. Dawes Sr.'s son! Both characters were played by Dick Van Dyke in the original movie, after all!
Perhaps Bert comes from a wealthy family, but decided to abandon a life of greed when he met Mary Poppins. It's also possible that Mary tried mend Bert's broken relationship with his father like she did with the Banks children, but ultimately failed because Mr. Dawes Sr. was too stubborn to change. That could be why she returns to Cherry Tree Lane every so often to check up on Bert. We don't know about you, but this completely changes the way we look at Mary and Bert's relationship!