Mary Poppins is hired as a nanny after the departure of Katie Nanna. She teaches the children valuable lessons in behavior, as they are portrayed as naughtier than in the film and books. She teaches them the value of looking past appearances when she takes them to see Bert. To illustrate the point, she animates the park statues. Later on, Mary Poppins takes the children to see their father at the bank. Though George is furious at first, the visit helps him realize just how much his values have changed and how much his kids mean to him. On the way home, Mary introduces Jane and Michael to the Bird Woman, as well as Mrs. Corry. That night, Mary warns Jane, who is in a temper after an outburst from her father, about controlling herself. As Mary leaves the room, the children are put on trial by their toys. When they are found guilty, Mary realizes that the Banks family has a lot to learn. To bring them to their senses, she decides to leave.
In the second act, Mary Poppins returns after the family suffers though the tyrannical behavior of Mrs. Andrews, George's former nanny. The two nannies have a face off, with Mary being victorious. Mary and the children have another adventure with Bert, who introduces them to his chimney sweep friends. Later, when George is called to the bank, Mary follows him with Jane and Michael in tow. At the end of the play, Mary leaves, knowing that the Banks family no longer needs her.