In the 1950s, a feral beagle/German Shepherd/terrier mix puppy wonders what life's true purpose is. Weeks later, while playing with his siblings, he is caught by dog catchers, is whisked away to the dog pound and is quickly euthanized, wondering if "fun" truly is life's purpose.
The dog's spirit is reborn in a newborn Golden Retriever puppy in 1961. He leaves his cage at a puppy mill and is found by two garbage men who plan to sell him. They leave to drink, leaving the dog locked inside their pick-up truck. He begins to die of heatstroke, but is then spotted by a kindhearted mother and her eight-year-old son Ethan Montgomery, who break the truck's window and rescue him. They bring him home and introduce him to Ethan's father. After they convince Ethan's father, the dog is allowed to stay and is named Bailey.
Bailey and Ethan bond quickly, especially over the summer when the family visits Ethan's maternal grandparents' farm, and Bailey now believes Ethan alone is his purpose. Years pass, and after several attempts at advancing in his job fail, Ethan's father has become a deadbeat alcoholic.
Over the summer, Ethan goes to a fair with Bailey and meets Hannah, whom he soon begins dating. Ethan and Hannah spend their summer together happily though their senior years are upcoming, but they plan to go to the same school; Ethan getting a football scholarship and Hannah following him on an academic one. When he goes home one night, Ethan's father is drunk and shouting at Ethan's mother. He turns on Ethan then and when his mother tries to keep them separated, he shoves her to the ground. Ethan orders his father to leave and never come back.
Later, at a football game watched by scouts, Ethan is offered a full scholarship to Michigan State, but that night a vindictive classmate named Todd throws a lit firecracker into Ethan's house, which results in a house fire. Bailey alerts Ethan, who goes to save his mother. Unable to get out of the door, they head out through his mother's bedroom window upstairs. Ethan first lowers her with her sheets to the neighbors below, then saves Bailey. However, he drops his rope and must jump, badly fracturing his leg and ending his athletic scholarship. Bailey attacks Todd, who is arrested by the police when firecrackers fall out of his pocket. Now, Ethan must go to an agricultural school where he will learn to take over his grandparents' farm. Falling into depression, Ethan breaks off his romance with Hannah before leaving for college, while Bailey stays with Ethan's grandparents. Shortly afterwards, Bailey becomes old and his health begins deteriorating. Ethan's grandparents take Bailey to the veterinarian's office, but Dr. Joyce is unable to save him, and Ethan comes into the office and watches as his beloved dog is put to sleep.
The moment he passes away, Bailey again is reborn into another dog life, a female German Shepherd puppy, who grows into a police dog named Ellie, sometime in the late 1970s/early 1980s, while possessing full memories of her past lives. Ellie is partnered with a lonely widowed officer named Carlos Ruiz, of the Chicago Police Department and works hard at "seeking" and "finding", now seeing the job as life's purpose. The two form a close relationship which ends after Ellie saves a kidnapped girl from drowning and is then fatally shot by the kidnapper. Carlos breaks down in tears as his precious dog dies in his arms.
Reborn again in the mid-1980s as a male Pembroke Welsh Corgi puppy, Bailey forms a bond with an Atlanta college student named Maya who names him Tino. Maya is lonely, and Tino makes it his purpose to find her some happiness. She starts a relationship with a classmate named Al whom she begins seeing after Tino falls for Al's dog, a black and white female Landseer named Roxy. Maya and Al marry and have 3 children. In love with Roxy and now seeing her as his purpose, Tino is heartbroken when one day Roxy does not return from the vet. As a now elderly Tino dies, he thanks Maya for giving him one of his best lives.