Ten-year-old Leigh Botts likes to read, loves his dog, Bandit, enjoys writing and is frustrated every day when he finds food stolen from his lunch box. As part of a second-grade school assignment, Leigh writes a fan letter to a popular author named Mr. Henshaw. Leigh chose this author because he likes the man's book, Ways to Amuse a Dog. In the fourth, fifth and sixth grades, Leigh continues to write to Mr. Henshaw. To Leigh's surprise, Mr. Henshaw writes back with advice about writing, relationships and getting to the bottom of the lunchbox thief.
Most of the letters are written when Leigh is in sixth grade, and has moved to a new school in a new town. Leigh is excited to hear from his favorite author and, at the request of his teacher, reads it to his classmates. At the end of the letter are 10 questions that Leigh does not want to answer, and he tells Mr. Henshaw as much. As a postscript, he includes the warning that if his dad were at home, he would tell Mr. Henshaw to "go climb a tree."
In his next letter, at the advice of his mother, Leigh decides to answer two of Mr. Henshaw's questions: Who are you? and What do you look like? Leigh describes himself as "plain." In the letter after that one, he answers the third question, What is your family like? Leigh explains that since his parents divorced, his father and Bandit are on the road most of the time. Mr. Botts drives a large truck transporting produce from state to state. Leigh says that the truck is the reason his parents split up and promises to tell more about his mother in the next letter.
In a letter dated the following day, Leigh writes to Mr. Henshaw that his mother works part time at Catering by Katy, where she fixes food for weddings and parties. He also mentions that Katy gives his mother delicious tidbits to put in his school lunch and that his mother is almost always there when he gets home from school.
The next letter is written a day later. In it, Leigh answers Mr. Henshaw's fourth question, Where do you live? Leigh explains that following the divorce, he and his mother moved to a house that is very small but at least "it can't be hauled away."
Several days later, Leigh writes to Mr. Henshaw again, although he says he would be watching television if it weren't broken. The fifth question is Do you have any pets? Leigh answers that, after the divorce, his father took Bandit to keep him company in the truck while he was on the road. Leigh confesses that he thinks about his dad and his dog a lot, and he misses them both.
The following day, Leigh answers questions six, seven, eight, nine and 10. Do you like school? Leigh says he figures that if he hangs in there, he'll get out of there. Who are your friends? Leigh doesn't feel that he has any real friends. Classmates just like to take the food from his lunch. Who is your favorite teacher? Leigh doesn't have a favorite teacher but likes the custodian. What bothers you? Leigh admits that he doesn't like being in an empty house and walks to school early after his mother goes to work. What do you wish? Leigh wishes that people would stop stealing the "good stuff" out of his lunch bag. He also wishes that his dad would come to his house with Bandit and take him to school in his truck so all of his classmates would be jealous.
Several days later, Leigh writes another letter to Mr. Henshaw, apologizing for his angry tone. He admits that maybe he was actually mad at his father for not sending child support and says that he's a little sad that he has answered all of the questions. He also reports that a delicious deviled egg was stolen from his lunch, and he was angry because he had looked forward to eating it.
Leigh's next letter mentions that he has followed Mr. Henshaw's advice and bought a composition book and labeled it "Private Diary — Keep Out." He is confused about the idea of keeping a diary and asks Mr. Henshaw for advice about what to write in it.
Later that month, Mr. Henshaw sends Leigh a postcard with a photograph of bears on the front, and Leigh writes him a thank-you note, acknowledging his advice to write in the diary as though it were a letter to someone. Leigh says that he will pretend he is writing to Mr. Henshaw and thanks him for the advice.
The next sections of the book are composed of diary entries addressed to "Dear Mr. Pretend Henshaw." In them, Leigh details his ideas about catching the lunch bag thief, which include writing a fictitious name on his lunch bag.
Over Christmas vacation, Leigh hopes that his dad will visit, but his mother reminds him that they are divorced. Instead of having a family holiday, Leigh and his mother enjoy a turkey dinner and a small Christmas tree. On Christmas Day, a trucker arrives with a present for Leigh that his dad arranged to be delivered — a hooded down jacket that Leigh wanted. After Christmas, Leigh writes in his diary that his dad phoned him from Oregon. He reports that the child support check has been sent and that he hopes Leigh liked the jacket he sent. Another entry notes that a small cheesecake was stolen from his lunch, and Leigh decides to wrap his lunch bag in Scotch tape so that no one can steal anything from it.
In a subsequent diary entry, Leigh relates a conversation with his mother regarding marriage. Leigh asks his mother why she doesn't get remarried, and his mother says that she is looking for a man who isn't in love with a truck. Leigh explains that he just wants a father at home who can show him how to make a burglar alarm for his lunch bag. His mother laughs and says that there must be a better way to solve that problem than for her to get married again.
Following the diary entries is a letter that Leigh decides to mail to Mr. Henshaw. He reports that the school librarian tracked him down so that he will be the first student to read Mr. Henshaw's new book. Leigh writes to Mr. Henshaw after finishing the book to say that it was wonderful, and he hopes that it wins a million awards.
Leigh confesses to his diary that he wishes his father would call when he says he would. He relates conversations he had with his mother about whether his dad likes them or his truck more. He worries that his dad and mother might still be married if he had never been born and is angry with his mother for divorcing his father.
At last Leigh's father calls, and Leigh is able to unload his heart. He talks about the person who has been stealing from his lunch bag and asks about Bandit. Leigh's father confesses that Bandit escaped from the truck and is lost. Leigh overhears a voice asking when the pizza will be delivered and suddenly feels invisible. Hurt, he hangs up on his dad.
Leigh is overwhelmed by the changes in his life. When he tells his mom that his dad lost the dog he loves and is now involved with someone else, Mrs. Botts patiently explains that their divorce is not his fault and that perhaps she grew up and Leigh's father did not.
Leigh decides to start taking his lunch to school in a black lunchbox instead of a paper bag so he can catch the lunch thief. He writes several letters to Mr. Henshaw asking questions about a story that Leigh started but doesn't know how to finish — a story about a 10-foot-tall man who melts as he drives a truck across the desert. In the next letter, Leigh acknowledges a letter that Mr. Henshaw wrote to him. He mentions an alarm that he rigged up to a black lunch box. Problems arise when lunch period comes and the alarm hasn't sounded, but Leigh needs to eat his lunch. The alarm goes off, and Leigh is proud.
When Leigh's dad phones, Leigh asks whether Bandit has been found — or whether another dog has taken Bandit's place. Mr. Botts responds that he is still looking for Bandit. Leigh admits that he misses his father. At the last minute, the librarian asks whether Leigh wrote anything for the Young Writers' Yearbook. When Leigh says that he has not, she encourages him to submit something. Leigh writes an account of traveling with his father in his father's truck.
Leigh invites Barry, a classmate, to his house for dinner and is happy to have a new friend. He summons the courage to ask his mother whether his dad will remarry. Mrs. Botts doesn't answer directly but says that Leigh's dad will never grow up. On the day that library awards are handed out, Leigh receives an Honorable Mention for his story "A Day on Dad's Rig." Leigh is disappointed not to have won first prize but is glad to see his name in print.
A few days later, the teacher in charge of the award asks whether Leigh would like to have lunch with Angela Badger, a renowned author. Having lunch with her is an award reserved for the winner. The actual winner copied the winning entry from a book of poems. Leigh accepts. Leigh attends the award dinner and is surprised when Mrs. Badger refers to him as an author. Mrs. Badger praises Leigh for not imitating others and being himself. Finally Leigh asks whether Mrs. Badger has ever met his hero, Mr. Henshaw. She says that she has. Later, Leigh wishes his dad had attended the dinner with him so they could talk about it.
In the final entries, Leigh reports that his father brings Bandit home. While his parents miss one another, it's clear that the two of them have parted ways. Leigh encourages his dad to take Bandit on the road so they can keep one another company.