A Dog's Purpose is told from the perspective of a dog, and describes the dog's adventures throughout multiple lives as he is reincarnated as a different breed with a new name, but possesses all the memories of his former lives.
We're introduced to the story by a homeless and perhaps wild dog, although he remains genteel throughout the story. This first dog, given the name Toby, ends up at a nice old woman's place called The Yard, where there are other dogs. However, after some time passes, men come and take all the dogs because of poor living conditions and aggressive dogs, and the narrator himself is put to sleep.
Life isn't over it seems, because our homeless dog comes back to life again and again, but as different breeds, and sometimes different genders.
At one point, the dog comes back as a golden retriever, who almost goes home with a truck driver, but is instead left in a hot car while the man goes into a bar. Luckily, a woman rescues Toby from the truck and gives him to her son Ethan, who loves Toby, and names him Bailey. Bailey has a pretty happy life with Ethan and his family, but one day as the years pass by, Bailey grows old and sick, and was put to sleep by a veterinarian to end his suffering.
But, once again Toby-Bailey is resurrected, and is now a female german shepherd, cared for by a police officer and named Ellie. Ellie is trained to help the officer when looking for missing people. After a while Ellie ends up working alongside the officer's partner, which is a pretty happy life. The partner, a woman, is set to marry her boyfriend, and Ellie is trained to be the ring-bearer. Ellie eventually dies.
Toby-Bailey-Ellie is reborn again as a lab, and has a couple owners and names in this life. A male again, he is given the name Bear by a woman who doesn't keep him for long because of a "no pets" rule where she lives. Bear goes to live with the woman's mother, but doesn't fare much better here than at The Yard. The boyfriend of the mother is a mean alcoholic and dumps Bear on the side of the road one day. To Bear's surprise, he realizes that he is close to one of his first owners, the little boy (now grown man) named Ethan. Bear's nose leads him to the house where Ethan lives, and has since married. Ethan keeps Bear, and names him Buddy. Now a lab retriever named Buddy, the dog is back with his former owner, and lives happily.
Unfortunately, one day when only Ethan and Buddy are home, Ethan falls ill and as he's dying, he begins to hallucinate that Buddy is actually Bailey (which would be correct). Ethan dies soon after, and Buddy remains dutifully by his side, waiting for Ethan's wife to return home.