Nick Allen isn't a bad kid, but he does have lots of ideas and enjoys tormenting his teachers with them. In third grade, he convinced Miss Deaver to let the class turn the classroom into a tropical island. Then, in fourth grade, Nick learned that blackbirds make high-pitched noises to confuse birds of prey, and noticed that his teacher, Mrs. Avery, looked like a hawk. He and one of his classmates, Janet Fisk, made high-pitched bird noises all year long to annoy Mrs. Avery—and she never caught them.
Entering fifth grade represents a turning point for Lincoln Elementary students. They no longer get recess, they get real letter grades, and they all get Mrs. Granger for language arts. Mrs. Granger has a reputation for being strict and assigning lots of homework, especially vocabulary homework. She also loves the dictionary. A few weeks before school begins, Nick's parents receive a letter from Mrs. Granger explaining that Nick needs to have access to a good dictionary for homework. Nick just groans.
On the first day of school, Mrs. Granger starts things off with a vocabulary test. Right before the period ends, Nick decides to deploy the "teacher-stopper," a question designed to keep teachers from assigning homework. He asks Mrs. Granger where the words in the dictionaries come from. Rather than tell him, Mrs. Granger asks Nick to research it himself and present his findings to the class the next day. After school, Nick grudgingly does Mrs. Granger's vocabulary homework and then turns to his report. Nick looks up "dictionary" in both the adult and children's encyclopedias and then comes up with one of his big ideas.
In Mrs. Granger's class the next day, Nick gives his presentation. Mrs. Granger loves it for the first eighteen minutes, but then she seems to understand that Nick is trying to drag his presentation out as long as possible. Finally, with ten minutes left in the period, she makes him stop. Nick doesn't stop there, however; he asks Mrs. Granger why words mean different things. Mrs. Granger explains that Nick and all other English speakers decide what words mean, and says that if everyone started to use a different word for something, it'd eventually end up in the dictionary.
That afternoon, Nick and Janet walk home together. Janet finds a gold pen in the street, but Nick is lost in thought about what Mrs. Granger said about words and why they mean what they do. He remembers that when he was a toddler, his parents knew he wanted to listen to music when he said "gwagala." Nick is so absorbed that he bumps into Janet and sends her pen flying. He apologizes, and as he hands Janet the pen, he calls it a frindle. The next afternoon, he stops at a shop and asks the saleslady for a frindle. She doesn't understand what he wants until he points to a pen. However, after six days of having kids ask for "frindles," the lady knows what Janet wants when she asks for a frindle. Nick and his friends take an oath to only use "frindle" and never say "pen" again.
The next day in Mrs. Granger's class, Nick and one of his friends make a show of Nick forgetting his frindle. The other kids laugh, but Mrs. Granger isn't amused. After class she asks Nick to not interrupt her with funny ideas. Nick looks innocent, insists he truly did forget his frindle, and promises to never forget his frindle again. Two days later, the school takes everyone's class photos. The fifth grade group photo is last. When the photographer asks the kids to say cheese, every child says "frindle!" and holds out a pen. Mrs. Granger is furious, but all the other kids at school think it's funny and start using "frindle." This leads Mrs. Granger to start giving kids detention for using "frindle," though detention with Mrs. Granger becomes a badge of honor rather than a punishment.
After a week or so, Mrs. Granger declares that "frindle" has gone far enough, but Nick notes that he's just putting Mrs. Granger's lessons on language into practice in the real world. He's not swayed when Mrs. Granger points out that the word "pen" has a rich history that makes sense, but Mrs. Granger seems unsurprised by his reaction. She asks Nick to sign and date the back of an envelope containing a letter for him, which she promises to deliver when this whole thing is over.
The next day, one of Nick's friends suggests that they get the entire class to individually ask Mrs. Granger for a frindle, reasoning that she can't keep everyone for detention. Mrs. Granger keeps 80 students that day and 200 the next. The principal, Mrs. Chatham, decides to visit Nick's parents to discuss the issue. Mrs. Chatham tells Mrs. and Mr. Allen her version of events, which is that the kids ruined the class photo and aren't respecting rules anymore. Mrs. Allen looks annoyed and states that the whole thing sounds like a gross overreaction to kids testing out a new word, but Mrs. Chatham says that they need to stop "frindle" for the same reason they need to keep children from using "ain't": standards. The adults are stumped when Nick points out that "ain't" is in the dictionary. Mrs. Allen repeats her point and soon, Mrs. Chatham leaves. Nick tells his parents that he never meant to be disrespectful and tells his dad that he can't make it stop—it's no longer just his word anymore.
The next day, a reporter named Judy Morgan hears that there's a revolt going on at Lincoln Elementary. The secretary, Mrs. Freed, is immediately annoyed when Judy asks to speak to someone about the "frindle" business but shows her in to see Mrs. Chatham. Mrs. Chatham tells Judy that it's a silly prank and an overreaction, but Judy can tell that Mrs. Chatham doesn't want to actually talk about it. Judy then goes to speak to Mrs. Granger. Mrs. Granger seems convinced that "frindle" will fall out of fashion soon and shares that a boy named Nick Allen started the whole thing. As Judy heads out to the parking lot, she runs into a group of students who just finished serving detention with Mrs. Granger. They're all excited to be part of a movement, but tell Judy that Nick probably won't want to talk to her. The next day, Judy receives an envelope at work containing the fifth grade class picture. Someone wrote on the back which kid is Nick.
On Thursday when The Westfield Gazette comes out, Judy's article about "frindle" is on the front page. The entire town is in an uproar. Kids in middle and high school start using the word, and Nick becomes a celebrity overnight. Bud Lawrence, a local businessman, begins selling pens with "frindle" printed on them and files a preliminary trademark on the word. A few days after the article comes out, Alice Lunderson, a CBS employee, reads Judy's article. It attracts the attention of the national CBS station in New York, and they give Alice permission to put together a piece for the evening news. Alice interviews Mrs. Granger, who insists that kids need to learn that language has rules, and then interviews the Allen family. Nick nervously explains that he was just testing out what he learned from Mrs. Granger and credits her for teaching him so much about words. After the segment airs, Nick is asked to give interviews on television and magazines.
Bud Lawrence's sales pick up, though his lawyer explains to him that now that everyone in the country knows that Nick invented the word "frindle," they'll need to make a deal with the Allen family to use the word. When Bud asks Mr. Allen to come to his office to chat, he sees that getting the rights to the word will be easy: Mr. Allen is overwhelmed and wants all the attention to stop. Mr. Allen agrees to sign a contract giving Nick 30 percent of the profits from frindle-branded merchandise and accepts a check for 2,250 dollars, Nick's cut of proceeds from the first week. Mr. Allen sets up a secret savings trust for Nick. Bud deposits bigger and bigger checks into Nick's account and the city council votes to put a "frindle" sign up in town. Mrs. Granger continues to insist that kids use "pen," but they all refuse.
Nick struggles to fully recover from his brush with fame. His ideas start to scare him a bit. When he learns about consumers, he thinks that he can use his new knowledge to get his classmates to boycott the cafeteria food until it improves, but he's too afraid of getting in trouble to tell anyone about it. He also thinks that Mrs. Granger forgot about the letter she wrote him. On the last day of school, he goes to ask her about it. She insists that the "frindle" thing isn't actually over yet, so he can't have the letter. She does tell him that she's actually proud of how he handled things and tells him to not be afraid of his big ideas. Mrs. Granger shakes Nick's hand and says she knows he'll do great things. This gives Nick the confidence to be proud of what he did. Two years later, he does convince students to boycott the cafeteria food, making Westfield's lunch program into the best in the state.
Ten years later, Nick turns 21 and gets control of the "frindle" savings account. He gives money to his family and then tries to forget about it. He also receives a package from Mrs. Granger. The package contains a new edition of Webster's College Dictionary, which has an entry for "frindle." It also contains Mrs. Granger's letter. It says that she's actually excited about Nick's new word and that she's choosing to play the villain. She writes that she loves the dictionary because it remains relevant even as things change, and it too can change and adapt. Also in the box is Mrs. Granger's favorite pen, with a note saying "frindle" clipped to it.
On Christmas morning, Mrs. Granger finds an official-looking envelope and a gift on her front porch. The envelope congratulates Mrs. Granger and explains that with a donation of one million dollars, a former student started a scholarship fund in her name. She thinks it must be a mistake, but turns to the gift. It contains a beautiful gold pen. The pen is engraved and says that Mrs. Granger can call the object whatever she wants. It's from Nick Allen.