Marguerite Henry's book is based on actual incidents in the life of a Grand Canyon burro. Brighty loves his life of independence along the Bright Angel Creek for which he was named, going to the rim in the summer and down to the canyon floor in winter. He is present as President Theodore Roosevelt discusses the beauty and grandeur of the Canyon and how it should be preserved for the American people.

Brighty is the first to walk the historic bridge connecting the north and south rims. Befriended by prospectors, government men and campers, Brighty alternately helps his friends and runs free as the spirit moves him. His friend and companion, Old Timer, a prospector, is murdered by a claim jumper. A saddened Brighty searches for the killer, an adventure which runs through the book.