Dear Mr. Henshaw begins with the book's main character, Leigh Botts, writing a letter, as part of a second grade classroom assignment, to his favorite author, Boyd Henshaw. Mr. Henshaw writes back with some questions for Leigh, and his mother demands he answer. Through Leigh's answers to Mr. Henshaw, we learn about personal matters, such as his struggles with his parents' divorce, his complex relationship with his father, his being the new kid in town, etc. Later, Mr. Henshaw encourages Leigh to keep a diary of his thoughts and feelings, and the book then switches from a letter format to a diary format.
Following the plot of the book, we see that Leigh Botts must learn to accept that he cannot change parts of his life; that, for example, his parents never will re-marry, people will continue to steal his lunch, and that he can never count on his father to be available when he is needed. We also learn that he must deal with problems that most young adults must deal with such as: feeling lonely because he is new in town, school assignments, etc. We learn that Leigh is sad or upset because of his complicated relationship with his father and, as a result, is mad that his father wasn't there for him when he was needed.