Winnie relates the events of her twelfth year and the many changes in her relationships and in her attitude toward growing up.

Winnie's celebrates her unique style through her first year with a double-digit age as she has many new experiences, from the high of spending time in New York City with her aunt and big sister, to the low of her two best friends suddenly becoming interested in boys.

As Winnie and her friends count the days until the beginning of high school, they attend a summer camp where they learn about sea turtles--as well as human relationships.

Eight-year-old Winnie is torn between being a good friend to her old horse, Chief, and unpopular friend, Simon, and wrangling an invitation to mean girl Tamson's birthday sleepover.

When Aunt Betty arrives at the Willis family's Wyoming ranch from California, Winnie is tasked with training her aunt's homesick horse Buck, something that Winnie finds hard to understand until she is invited to her first sleepover.