Nick Allen is a basically good kid with an exceptional imagination. He is known for elaborate time wasting questions that can take up most of a class, but heading into fifth grade, Nick realizes that pulling off some of his more elaborate stunts is going to be more challenging, especially now that he and his classmates will be receiving a graded report card, and especially since they are going to have Mrs Granger for language arts. Mrs Granger has a fearsome reputation and an obsession with widening her students' vocabulary. At the start of the year Mrs Granger sends a letter home with every student explaining that every home is expected to have a dictionary which pleases his parents but makes Nick realizes that he is not going to enjoy fifth grade as much as he enjoyed the grades that preceded it.
It does not take long for Nick to find out that Mrs Granger is immune to the time-waster question; when he asks whether all of the dictionaries just copy words from each other, she asks him to research the subject and write a report to read aloud in class the next day. Not only does Nick have homework to do but he also misses out on a friendly game of baseball down the street because his research takes up most of the evening. However, he devises a plan to make the report itself be the time-waster and writes a report so lengthy that reading it aloud takes up over half of the language arts period.
Mrs Granger tells Nick that words mean what people decide they mean and that is how a language is born. This tidbit stays in Nick's mind and he decides to introduce a new word himself. Whilst walking home with his friend Janet, he picks up the gold pen she has just found and says, "Here's your frindle." The plan took shape that night at home and the next day Nick goes to the local store and asks for a frindle. Each day for five days, Nick sends a friend into the store asking to purchase a frindle until on day six, when Janet goes in to ask for a frindle, the lady behind the counter does not even ask what she means, merely asking if she wants blue or black. Nick gathers the kids together and they agree that fr this day on they will never use the word PEN again it always refer to it as a FRINDLE.
The following day, Nick raises his hand to tell Mrs Granger that he has forgotten his frindle. Three rows away another child calls out that he has a couple of frindles in his backpack and would be happy to lend one to Nick. Mrs Granger warns him about disrupting her class. When class pictures are taken two days later, instead of saying "cheese" when the photographer instructs them to, every fifth grader says "frindle" and holds one up on camera. As the photographer had no more film with him this was the fifth grade picture for the year. Several teachers were not very happy about this and Mrs Granger was absolutely furious. Although nobody had intended to make the teachers angry everyone in the s school enjoyed using the new word and found it great fun. Mrs Granger did not find it fun. She posted up a notice on the bulletin board that said anyone heard using the word frindle instead of the word pen would be kept after school and made to write "I am writing this punishment with a pen" one hundred times. This seemed to have the opposite of her desired effect as it just made everyone want to use the word even more. For weeks there were kids in her classroom after school until Mrs Granger called Nick into her classroom and asked him to stop the children from using his word. Nick says that he can't as he and his friends swore an oath to keep using it. Then Mrs Granger becomes a little mysterious. She pulls a letter that she has written to him out of her desk drawer and makes him sign his name and write the date on the back of the envelope, so that when he does eventually come to read it he will know it is the same letter.
When over two hundred kids end up in Mrs Granger's classroom after school on the same day, parents begin to call the principal to complain. The principal, Mrs Chatham, decides to pay a visit to Nick's parents at home.
she explains what has been happening at school and Nick's parents listen carefully, but his mother feels that the whole thing is rather a fuss over something pretty silly. Mrs Chatham says that Mrs Granger feels that there should be standards, that kids can't be walking around using words like "ain't" and at that, Nick pipes up that the word ain't is a Rialto in the big classroom dictionary that Mrs Granger says is law. Nick's mother still feels the issue is not Nick and his new word but Mrs Granger's over-reaction to it.
It is a rather slow news week at the local newspaper and when reporter Judy Morgan hears about the uproar around Nick's word she decides that there might be a story in it. She goes to the school and speaks with Mrs Chatham who is very uncomfortable and admits that Mrs Granger overreacted a little bit. Mrs Granger tells her the whole thing was very silly and that the instigator was Nicholas Allen. Then she talks to some of the kids outside who don't seem to mind being kept after school. They still want to use their word. After her article is published even high school kids start to use Nick's word and he becomes something of a folk hero. Local businessman Bud Lawrence is also rather impressed and trademarks the word frindle which he then sets about imprinting on cheap plastic ballpoint pens and selling for a rather nice profit.
The frindle story would have died out naturally on its own if not for a television reporter who read about Nick in The Westfield Gazette and decides to send the story to the network in New York. They interview a rather disapproving Mrs Granger then go to talk with Nick and his mother who are careful not to say anything controversial. The following day the report airs on the CBS evening news. Nick is wanted for interviews with all of the most popular television shows and in all the most popular magazines and the word frindle spreads nationwide. Bud Lawrence's attorney realizes that the whole country now knows that Nick made up the woes and suggests that in order to avoid a lawsuit he offers Nick a cut of his profits. He signs a deal with Nick's dad that gives Nick royalties on every sale. Sales are strong and Westfield even adds "Home of the original frindle" to the sign on Route 302.
Nick is more subdued for the rest of the year, wary of doing anything that might create the same waves. He remembers the letter Mrs Granger made him sign and goes to collect it from her but she tells him it is not time yet. Ten years later, Nick is a junior in college and is very very rich thanks to all of the royalties that were sitting in a trust fund for him. Another important thing that happens is a package that arrives from Mrs Granger that includes the letter, a short note and a new dictionary. The note instructs him to turn to page 541, where he finds the word frindle, alongside a definition. The letter tells him that this is the dictionary Mrs Granger now uses to teach her students new words and that when they ask how words are added to the dictionary she tells them all about the word frindle. The letter he had signed all those years ago was a congratulation note she had penned and planned to mail as soon as Nick's word was accepted into the dictionary.
Nick realizes that she had been rooting for frindle all along. About a month later Mrs Granger also received a package telling her that someone had set up a scholarship find in her name. It was accompanied by a beautiful gold fountain pen with a note from Nick saying that she may call the object whatever she chooses.