In a charming town during the year 1909, Jim Dear (Thomas Mann) surprises his beloved wife Darling (Kiersey Clemons) with an adorable cocker spaniel puppy named Lady (Tessa Thompson) as a delightful Christmas gift. As Lady flourishes under the loving care of her owners, she forms deep friendships with Jock (Ashley Jensen) the Scottie, and Trusty (Sam Elliott), the wise elderly Bloodhound who resides nearby.
Across town, a street-savvy terrier-mix named Tramp (Justin Theroux) is busy navigating the urban environment, scavenging for food, and deftly avoiding the relentless local dogcatcher. His mischievous antics frequently involve rescuing his buddies, Peg (Janelle Monáe) the spirited Pekingese and Bull (Benedict Wong), the tough Bulldog, from the grips of canine confinement.
One pivotal day, Tramp overhears a concerning conversation between Lady, Jock, and Trusty about their owner's changing demeanor. He learns that Darling's pregnancy has left her feeling neglected, and with a sense of urgency, he intrudes into their discussion. Claiming to speak from experience, Tramp warns Lady that with the arrival of a baby, a dog's position in the family may diminish significantly. This revelation infuriates Jock, who chases Tramp away in protective anger.
Tramp’s ominous warning plunges Lady into a spiral of anxiety throughout Darling's pregnancy, but when the new baby boy finally arrives, she finds comfort in being able to bond with her little sibling, alleviating her worries.
Afterward, with Jim Dear and Darling heading off on a brief trip, they leave their household, including Lady and the infant, in the care of Aunt Sarah (Yvette Nicole Brown), who has a strong dislike for dogs. She forbids Lady from seeing the baby and introduces her two troublesome Siamese cats, Si and Am. With the cats wreaking chaos, they cleverly frame Lady for their mischief.
In a dramatic twist, Aunt Sarah confines Lady and muzzles her at the local pet shop. Panic ensues as Lady escapes into the streets, where she is pursued by three fierce dogs until Tramp arrives just in time to save her.
Tramp takes Lady to the zoo, where a friendly beaver frees her from the muzzle, unleashing a thrilling exploration of the town. Their escapade leads them to Tony's Restaurant, where they share a delectable spaghetti dinner, followed by a romantic stroll in the park.
The next day, Tramp urges Lady to embrace his carefree lifestyle, but feeling a sense of responsibility, she declines. Though feelings have developed between them, Lady prioritizes her duty to protect the baby. As Tramp walks her home, his chicken-chasing antics draw attention from the dogcatcher, resulting in Lady being caught and taken to the pound, where she reunites with Peg, Bull, and various other strays who are well-acquainted with her charming yet roguish companion, Tramp.
The stray dogs regale Lady with tales of Tramp's flirtatious history, shedding light on his tendency to attract females, a revelation that instigates Lady's journey toward self-reflection. When Aunt Sarah reappears, she chains Lady in the backyard as punishment for her escape attempt. In a perhaps misguided display of devotion, Jock and Trusty propose a radical idea: Lady should marry one of them to escape her predicament. However, Lady, valuing her independence, graciously turns down their proposal.
As tensions rise, Lady becomes increasingly agitated when she notices a large rodent infiltrating the baby’s bedroom. Her desperate attempts to alert Sarah remain ignored until Tramp returns, drawn by her frantic barking. He bravely confronts the intruder behind a curtain, emerging victorious but injured.
With the immediate threat neutralized, Lady seizes her chance to burst free from her chains and rushes inside. During the ensuing chaos, the baby's crib tips over, resulting in the infant's cries. Aunt Sarah arrives, mistakenly believing the dogs have harmed the baby, while the reality is far from her understanding.
Meanwhile, when Jim Dear and Darling come home, they discover Sarah has imprisoned Lady in the cellar and has turned Tramp over to the dogcatcher. Disbelieving Sarah's claims, Jim Dear releases Lady, who reveals the truth about the rat’s fate. As the family contends with the brewing chaos, Jock and Trusty race off to save Tramp from the dogcatcher, but their rescue attempt is foiled when the wagon gets spooked, crashing in the ruckus.
In a heartening conclusion, Jim Dear and Darling arrive just in time to save Tramp from his dire fate. In an endearing twist, Tramp earns his place in the family, and he and Lady joyfully welcome four delightful puppies into their lives. Upon receiving a visit from Jock, who is now accompanied by a recovering Trusty, the family is filled with laughter as Trusty, in a charmingly forgetful moment, hilariously misplaces his old tales, leaving everyone in joyous fits of laughter.
