Paddy O'Day is an eight-year-old Irish girl who travels to America to join her mother, a cook for a wealthy family. On the voyage she befriends two Russian performers, Tamara Petrovitch and Mischa Petrovitch, and even participates in a dance with them, all while trying to sneak her dog Tim along.
Upon arrival at Ellis Island, officials inform Paddy that her mother has died and are prepared to send her back to Ireland. To keep her calm, they tell her that her mother is very sick, and move her to a locked dormitory on the island. Paddy escapes from the dormitory and hides with her dog Tim in an empty milk canister, a small act of bravery that sets the rest of her journey in motion.
The milk truck carries Paddy to Manhattan, where she emerges into a world of noise and traffic that overwhelms her at first. A band of street urchins accosts her, but she defends herself with the help of Tim. A kindly policeman intervenes, stops a speeding car, and arranges for Paddy to be taken to Long Island so she can be reunited with the family she believed she had left behind, if only temporarily.
Back at the house where Paddy's mother worked, the staff go to great lengths to keep the child hidden from the two elderly, particular aunts who supervise the household, Aunt Flora and Aunt Jane. Dora Dora the maid breaks the devastating news that Paddy's mother has died. Paddy's dog Tim slips away, chases the aunts' cat Mathilda, and forces Paddy to scramble for safety until she can be found. She finally hides in the room of Roy, the reclusive young master who overlooks a taxidermy collection of birds, and she charms him with a mix of mischief and sweetness. Even though Tim destroys one of Roy's avian models, Roy still takes a liking to Paddy and agrees to keep her presence a secret from his aunts.
Tamara and Mischa return to seek Paddy out, and they persuade Roy to have Paddy live with them. Seeing the wealth of the household and the companionship of his new friends, Roy loosens up from his eccentric, shut-in persona, grows a mustache, and starts playing the guitar in a colorful Russian-inspired style, even adopting a lifestyle that his aunts find shocking. Mischa's nightclub ambitions come into focus as he stages a show that features Paddy performing a bright song and dance in the number "I Like Balalaika" with a troupe of balalaika players, while Tamara dances a traditional Russian routine. The private detective engaged by the aunts alerts Immigration authorities, and Tom McGuire arrives to arrest Paddy. In a final twist, Roy announces that he and Tamara were secretly married the day before and that they will adopt Paddy, ensuring that the girl will stay with family who loves her. The story closes on a note of hopeful adaptation, as Paddy looks forward to a future filled with music, companionship, and a sense of belonging.
