This is a somewhat fictionalized account of the book about the true adventures of Nick Wilson called "The White Indian Boy" or "Among the Shoshones" depending on which edition you read. Both titles have the same text.
The book was more thorough and funnier but covers a longer time frame than this movie.
Basically, this movie describes the adventures of a 12 year old pioneer boy who runs away from his family and the drudgery of work on a frontier farm. He is lured away by Indians who promise him a horse if he will consent to be adopted by the Indian Chief's mother as the white child she dreamed she would have to replace the two other sons she has just lost.
He then lives and travels with the Shoshone Indians of Wyoming, Utah, and Idaho for 2 years. He becomes the adopted brother of the famous Chief Washakie and learns Indian skills from hunting to fighting.
Karen Allen, the best known actor in the film, is listed as the star but she really only has a small part. The main character is Nick Wilson with strong and authentic supporting roles by the Shoshone Indian actors. After 2 years, Nick returns home and meets up with his brother who has been searching for him all these years.
The main strength of this movie is the sympathetic portrayal of the American Indian. One weakness of this movie is that Indian life is idealized, much like Kevin Costner in "Dances With Wolves". The book does a much better job pointing out the bad as well as the good of Indian culture.
I would recommend this as a solid family film and would recommend reading the book after for a fuller and more true account.