With his tribe's move to follow the migrating bison, Yakari, the little Sioux boy, sets out on his own to follow the trail of Little Thunder, a wild mustang said to be untameable. Travelling far from home and deep into the territory of the terrible cougarskins, Yakari and Little Thunder undertake a great adventure and find their way back home.
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.