Fluke is a mutt puppy (a wrong colored Golden Retriever) who has flashing memories and dreams of having lived a human life. After being taken to a pound and eventually escaping, he is raised by an elderly homeless woman named Bella, who gives him the name 'Fluke', stating that he is a "fluke by nature, Fluke by name." Fluke supports Bella by helping her earn money from passing strangers, who are impressed with Fluke's ability to beat Bella's shell game.After Bella dies of an illness due to poor conditions, Fluke meets a street-wise dog named Rumbo who takes him to see a man named Bert who feeds Fluke and Rumbo. Fluke matures into an adult dog and eventually realizes that he was once a man named Thomas P. Johnson, who died in a car crash.Fluke is then abducted by a man named Sylvester to be used in makeup experiments at a cosmetics company. During his captivity, Rumbo comes to the rescue, but is shot by Sylvester (after taking a gun from a security guard who refused to shoot the dogs) as he and Fluke escape. A dying Rumbo tells Fluke that the black-and-white snapshot of a man in a sailor suit on Bert's wall was him and Bert was his brother and that he wishes to smell the sea again, suggesting that he died in the line of duty.After Rumbo's death, Fluke seeks out his surviving wife Carol and son Brian and reunites with them by becoming their new family dog. Though Carol, (implied to be afraid of dogs), is apprehensive about adopting Fluke, she caves in seeing how Brian has quickly bonded with Fluke. During his life as a dog, Fluke gets to know his family better and bitterly realizes, much to his regret, that he had been a distant workaholic.As more memories return, Fluke suspects that his human death was caused by his former business partner Jeff Newman. Enraged by this and the fact that Jeff is now dating Carol, Fluke viciously attacks him when he visits the house, and ends up getting put outside. Jeff calls for Animal Control against the protests of Brian, and so Fluke is forced to run from the premises.The next night, Brian goes missing while looking for Fluke and Carol implores Jeff to help, so Jeff drives back to their house. Fluke, who had been hiding in the backseat of Jeff's car while the latter was at work, comes close to killing Jeff by causing him to get into a car accident like his own. Fluke then has another flashback and realizes that Jeff wasn't responsible for his death. Instead, Fluke had caused his own death when he recklessly drove on the wrong side of the road just to argue with Jeff one night and swerved off to avoid hitting an oncoming truck. Jeff later tried to save him but failed. An injured Jeff, implied to have realized Fluke's true identity but bearing no ill will, tells Fluke to go find Brian before the latter catches hypothermia from the falling snow. Regretful over his actions, Fluke barks for a passing driver to help Jeff before running off.On a hunch, Fluke goes to the graveyard where he had been buried and finds Brian there, who had been locked in by an unaware groundskeeper. Fluke huddles with Brian to keep him warm. Carol, having come to the same conclusion from one Brian's drawings of his father, uses her car to break open the cemetery gates and picks up Brian. Brian tells Carol that he heard Fluke tell him in a vision that Fluke had to leave them. Carol tries to coax Fluke to come home with them. Instead, Fluke digs away at the snow in front of his tombstone to show Carol who he really is by uncovering the word "forever" at the bottom, a phrase he often said to her as a human. Carol is left speechless, and lets Fluke leave without objection. With a heavy heart, Fluke departs and entrusts his family to Jeff for their happiness. He monologues that he had to leave because he finally accepted that he can no longer be the family man he should have been, and that he should just cherish the life he has now, which he had not done back then.Far away and sometime later, Fluke is resting under a tree on a farm by himself. To his surprise and happiness, he is reunited with Rumbo, now reincarnated as a squirrel. Rumbo tells Fluke about life as a squirrel and about reincarnation.