Gertie's Leap to Greatness tells the story of Gertie Foy: a fifth grader determined to be the best fifth grader in the world. Living with Aunt Rae and her father Frank, Gertie was abandoned by her mother Rachel. Gertie’s school bus takes her by Rachel’s house every day. One day, Gertie sees that Rachel’s house is for sale and Gertie decides that she must become the greatest fifth grader in order to impress Rachel.

Gertie tells her friends Junior and Jean about her plans for greatness. She brings a zombie frog for show-and-tell and feels certain that she will have the best story. School gets off to a bumpy start for Gertie when Mary Sue, a new girl, ends up taking Gertie’s seat and stealing Gertie’s attention. Immediately, Gertie finds herself in competition with Mary Sue. While Mary Sue does not engage in the competition at first, she does recognize that Gertie is jealous of her.

Mary Sue finds her new school beneath her, telling her new classmates that she learned her state capitals the year before when she was living in California. Mary Sue brags about her father being a famous film director and how she is friends with Jessica Walsh, a popular young film star.

Gertie sees Mary Sue as the competition she must defeat in order to the be the best fifth grader. Gertie’s friend Jean expresses doubts about Gertie’s ability to beat Mary Sue, because Jean does not think that Gertie is smart enough to do so. This causes a rift between the girls and Gertie comes to rely more on her friend Junior.

When Mary Sue starts an environmental awareness club that excludes Gertie because Gertie’s father works on a oil rig, Gertie decides to infiltrate the party with Junior’s help. Junior sneaks into the party while Gertie waits outside. When Gertie is unable to see Junior anywhere in the party, Gertie sneaks in as well.

The competition between Gertie and Mary Sue continues when it is revealed that there will be a school play and both girls want the lead. Mary Sue feels she deserves the part because she considers herself an actress, but Gertie blows Mary Sue away during the audition. Gertie is excited to get the part and ventures out on her own to go and tell Rachel about the play. When Gertie arrives at Rachel’s, Gertie sees that Rachel and her new family are having a birthday party. Gertie is crushed, but invites Rachel to come see her in the play anyway. Aunt Rae rescues Gertie from Rachel’s and takes her home.

Gertie begins to act out and ends up stealing a dish of chocolates from the principal’s office and eating all of the chocolates during recess. Gertie’s actions cost her the lead in the play and Gertie is further hurt by her part going to Mary Sue. When the night of play arrives, Gertie is helping backstage when she is asked to find Mary Sue, who is no where to be found. Gertie finds Mary Sue crying in the bathroom because she feels like her father cares more about Jessica Walsh than his own daughter. Mary Sue reveals that her parents are divorcing and Mary Sue and her mother will be staying permanently. Gertie encourages Mary Sue to perform in the play and watches from the audience. Rachel never arrives.