An ex-police/army dog (German Shepherd), named Rex inherits a fortune from an eccentric millionaire. But someone poisons him for his fortune, and he gets to go back to earth as a human detective to bring his killer to justice, and protect the girl who used to look after him.

When tycoon Andrew Lindsay dies, he shocks his community by leaving his six million dollar fortune to King, his faithful dog. Lindsay's secretary, the sweet and lovely Ellen Hathaway, is the trustee of the estate and next in line to inherit. Ellen is besieged by fortune hunters, and when Corporal Perry Collins comes by to visit King and King seems to dislike him, she suspects that he is just another fortune hunter. Perry soon wins her over, however, by regaling her with stories of his days with King in the Army K-9 Corps. Weeks later, Ellen is falling in love with Perry, but King remains wary. Perry leaves for a business meeting, promising that if he secures backing for his project, he will return to ask Ellen "an important question." Later that day, King is found dead from poisoning, and the police immediately name Ellen as their top suspect. King's ghost rises that night to Bestiatory, or animal purgatory, where each spirit is judged and either allowed into heaven or sentenced to return to earth as a "hum-animal." Hum-animals are humans who retain their animal tendencies, and they must remain as such until they have paid for their former sins. King asks for, and is granted special permission to go back as a conscious hum-animal in order to clear Ellen of the false charges against her. With a guide, a female racehorse named Golden Harvest, King returns to earth as Rex Shepard, a private investigator with a taste for kibble. He informs Goldie that Perry is a sadistic animal abuser and killed King in order to marry Ellen and swindle her out of her inheritance. Rex's first order of business is to visit his old home, but Ellen at first refuses his offer to help her clear her name. She relaxes, however, when Lindsay's other dog, Tillie, who has not eaten since King died, joyfully eats out of Rex's hand. The next day, Perry informs Rex that he has persuaded Ellen to drop his investigation of the case, after which Rex sneaks back to the house to reconstruct the night of the murder from memory. He soon digs up the bottle of poison Perry used, and when Ellen sees it, she re-hires him. As soon as Rex asks her to postpone her upcoming wedding to Perry, however, she assumes he is only after her money and turns him in to the police as a possible murder suspect. The police bug his office, but as soon as they hear Rex reminiscing with Goldie about his days as a dog, they decide he is merely insane. Now unable to procure the police's help, Rex tracks down his old K-9 Corps friend, a bulldog named Max. He steals Max from his kennel and brings him to Ellen's, where he informs her that Perry beat the Army dogs, and proves his claim by setting Max loose to attack his old corporal. When Max's owner, Perry's old commander Major Smith, arrives, he confirms that Perry was transferred because he was cruel to the animals. With Perry safely in police custody, Ellen asks a smitten Rex to visit her again, but Goldie reminds him that they must return to Bestiatory that night or be forced to remain on earth as humans with no memory of their animal days. Rex at first leaves with Goldie, but when she mentions that Ellen is actually a part-shepherd hum-animal, Rex decides to stay on earth with Ellen for at least one more lifetime.