In the U. S. Army intelligence office, bumbling lieutenant Peter Stirling receives a coded message from his friend, Francis, a talking mule. The note urges Pete to hurry to the Coronado, California naval base, where Francis is about to be sold as surplus. Pete rushes to the train station, but before he can board, nurse Betsy Donevan mistakes him for her shell-shocked brother, Navy boatswain Slicker Donevan. She tries to forcibly remove his uniform so he will not get into trouble for impersonating an Army officer. Finally she realizes that Pete is not Slicker but merely his mirror image.