John Emery and James Millicam are brothers, and two different personalities. Emery is a stuffy physicist who had an important position in the development of the atomic bomb, and is now working on some sort governmental-sounding agencies. Millicam is a globetrotting explorer. They don't get along well, but when Millicam is reported killed in a plane crash, Emery has mixed feelings. So when a large, scruffy mutt wanders into his life and he says it's Millicam reincarnated, people around him think he's dealing with grief in a peculiar manner. Except that his idea becomes fixed.
It's a comedy, and while Emery's behavior is appropriate for a character who is going to get kicked in the pants, it doe have some clever lines in it. And he does get kicked in the pants emotionally, when the dog disappears, only to turn up in the possession of Hillary Brookes. She has been forewarned about Emory's delusion -- if it is a delusion; you can never be sure in movies of this period -- and resists his taking her dog.
By the way, the dog, which is called a mutt throughout, is actually a faun Berger Picard, an unusual, though fully recognized breed of sheepdog. Because of their scruffy-looking appearance, they turn up in a lot of movies as mongrels. With Diana Douglas, Charles Brown, Percy Helton, and Jeff Corey.
