Teenage Corky helps her aunt and uncle train wild mustangs for the horses' owners. This year she gets to train a mustang on her own. Challenged by the difficulties of training the feisty mare she names Glitter, Corky often feels her life is like riding a roller coaster with no brakes on uneven tracks. While working with Glitter, Corky must also deal with her own shortcomings, like taming her quick temper. This is evident after she bloodies the nose of the new boy at school for taunting her and calling her "mustang girl."

Corky is determined to show everyone she has what it takes to become a real horse trainer in her own right, but does she? Can she train the mustang according to the owner's unusual demands? Or will her beloved Glitter be doomed to face the owner's rejection—and possibly the slaughter auction?