Decades after her original visit, the magical nanny reappears to once again help the Banks siblings, Michael and Jane, now adults facing their own personal crises. Michael, a widower with three young children, is struggling to keep both his family and his childhood home together amidst the imminent threat of repossession by the bank where he works. The family's bereavement has left them with a sense of hopelessness, and the joy and wonder once present in the household have been overshadowed by the anxieties of life's harsh realities.

As financial woes and the pressure to find a missing certificate that proves valuable shares owned by the family intensify, the enigmatic caretaker returns, floating into their lives carried by the east wind. Accompanying her is Jack, a good-hearted lamplighter who knew Mary from before. With her unique blend of whimsy, wisdom, and wit, she sets about helping the Banks children, both young and grown, rediscover the joy and wonder missing in their lives through a series of fantastical adventures, including an underwater escapade and a journey across the city’s skyline. Her unorthodox methods and magical interventions begin to remind the family that it's possible to face life's difficulties with a sense of imagination and fun.