At the start of fifth grade, Nicholas "Nick" Allen is unhappy because his English teacher is the much-disliked Mrs. Granger. One day, to stall for time in class, Nick decides to question where each word comes from. After hearing Mrs. Granger's explanation, and having to write an essay about it for homework, he creates a new name for the pen: "frindle". Soon, every child in school is using the word, and it gains national attention. Mrs. Granger dislikes this new word because she thinks it is not respectful to the word pen, which has a long history. She makes children stay after school and write lines for saying the word "frindle", but this proves to be a problem, as almost every pupil has to stay after school. Parents complain, and the bus drivers that have to work overtime are also unhappy. The principal decides to visit Nick's house to end the use of the word, but the situation is out of Nick's hands now, and the word's usage cannot be controlled. The epilogue shows Nick as an adult in his own house. He becomes very rich from the fund of his dad's longtime friend, and Mrs. Granger sends him a new copy of the Dictionary, recently updated to include new words, including "Frindle".

Nick is not a good kid or a bad kid or a really smart kid; Nick is an idea kid. He's also the best class interrupter in the whole school - until he gets to Mrs. Granger's 5th-grade language arts class. A simple question about the dictionaries that Mrs. Granger loves - meant to distract her from giving the night's homework - leads to an oral report by Nick on the history of the dictionary and how words are created. What at first seems like Nick's defeat soon leads to his best idea ever: He will create a word. Which is how Nick and his friends start calling their pens "frindles." Mrs. Granger refuses to accept the new word for pen, which has been quickly adopted by the entire 5th-grade class, fighting against the 5th graders every step of the way. Once the whole school gets involved though, Nick begins to realize that while he started the "frindle" movement, it's now far beyond his control.

"Ten-year-old Nick Allen has a reputation of devising clever, time wasting schemes guaranteed to distract even the most conscientious teacher diversions backfire in Mrs. Granger's fifth grade class, however, resulting in Nick being assigned a report on how new entries are added to the dictionary. Surprisingly, the research provides Nick with the best idea ever, and he decides to coin his own new word. Mrs. Granger has a passion for vocabulary and Nick's ( and soon the rest of the school's ) insistence on referring to pens as "frindles" annoys her." ( Novelist review)