On Christmas morning in 1909, Jim Dear gives his wife Darling a cocker spaniel puppy that they name Lady. Lady enjoys a happy life with the couple and with a pair of dogs from the neighborhood, a Scottish Terrier named Jock and a bloodhound named Trusty. Meanwhile, across town by the railway, a stray silver mutt, referred to as the Tramp, lives life from moment to moment.
Later, Lady is saddened after Jim Dear and Darling begin treating her rather coldly. Jock and Trusty visit her, and determine that the change in behavior is due to Darling expecting a baby. While Jock and Trusty try to explain what a baby is, the eavesdropping Tramp enters the conversation and offers his own opinions. Jock and Trusty take an immediate dislike to the stray and order him out of the yard as Tramp leaves, he makes a final speech that when the baby moves in, the dog moves out.
In due course, the baby arrives and Jim Dear and Darling introduce Lady to the infant. Soon after, Jim Dear and Darling decide to go on a trip together, leaving their Aunt Sarah to look after the baby and the house. When Lady clashes with Aunt Saraha’s two Siamese cats, Si and Am, she takes Lady to a pet shop to get a muzzle. A terrified Lady escapes, but is pursued by some street dogs. Tramp sees the chase and rescues Lady. The two then visit a zoo, where Tramp tricks a beaver into removing the muzzle. That night, Tramp shows Lady how he lives footloose and collar-free, culminating in a candlelit Italian dinner.