A pet cocker spaniel falls in love with a jaunty mutt who protects her.
On Christmas morning in 1908, Jim Dear gives his wife Darling a cocker spaniel puppy that they name her Lady. When Lady grows up, she 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 friendly stray silver mutt, referred to as Tramp, he dreams to live in a family and home, be it begging for scraps from an Italian restaurant or protecting his fellow strays Peg (a Pekingese) and Bull (a Bulldog) from the local dog catcher. At one point Lady sees a rat trying to sneak into the yard and chases it away.
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.
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 Sarah'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 alley 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 spaghetti dinner.
As Tramp escorts Lady back home, he do one last thing is to chase hens in a chicken coop and then he is through being a stray. When the two dogs flee, Lady is caught by the dog-catcher. At the pound, the other dogs admire Lady's license, as it is her way out of the pound. The other dogs also reveal that Tramp's had multiple girlfriends in the past, and they feel it's unlikely that he'll ever settle down. Eventually, Lady is collected by Aunt Sarah, who chains Lady to a doghouse in the back yard. Jock and Trusty visit to comfort her and tell her that strays are not allowed to be adopted. When the Tramp arrives to apologize, thunder starts to rumble as Lady angrily confronts him, after which the Tramp sadly leaves.
Moments later, as it starts to rain, Lady sees the same rat from before trying to sneak into the yard again. While the rat is afraid of Lady, it is able to evade her and enter the house. Lady barks frantically, but Aunt Sarah yells at her to be quiet. The Tramp hears her and runs back to help. Tramp enters the house and finds the rat in the nursery. Lady breaks free and races to the nursery to find the rat on the baby's crib, as it had intended on killing the baby. Tramp pounces on the rat, but knocks over the crib in the process, awakening the infant. Tramp kills the rat, but when Aunt Sarah comes to the baby's aid, she sees the two dogs and thinks they are responsible. She pushes Tramp into a closet and Lady into the basement, then calls the pound to take the Tramp away.
Jim Dear and Darling return as the dogcatcher departs. They release Lady, who leads them to the dead rat, vindicating Tramp. Jock and Trusty, having overheard everything, chase after the dog catcher's wagon. Jock is convinced Trusty has long since lost his sense of smell, but the old bloodhound is able to find the wagon. They bark at the horses, who rear up and topple the wagon onto a telephone pole. Jim Dear arrives by car with Lady, and Lady is happily reunited with the Tramp before they discover that the wagon fell on Trusty.
That Christmas, Tramp, now a part of Lady's family, has his own collar and license. Lady and the Tramp raise four puppies together: three resemble Lady (Annette, Collette, and Danielle) and the other resembles Tramp (Scamp). Jock comes to see the family along with Trusty, who is carefully walking on his still-mending leg. Tramp is happy to have finally become a house dog, and he and Lady live together happily.