The story is about Leigh Botts, a young boy who lives with his divorced mother and misses his father. The beginning of the book is a collection of letters written from Leigh to Mr. Henshaw, his favorite author.
Although we never see Mr. Henshaw's reply, Mr. Henshaw apparently answers at least two of Leigh's letters and suggests that a would-be writer should write in a diary every day. At first Leigh's diary entries take the form of unsent additional letters to Mr. Henshaw, even beginning with the salutation, "Dear Mr. Pretend Henshaw." But as he matures, Leigh eventually decides that he does not need to do this and begins keeping his diary only for himself. The story begins with the main character in the second grade but he progresses to sixth grade in less than 10 pages.
The diary reveals Leigh's loneliness at school, details his troubles with an unknown schoolmate who secretly steals his lunch in the mornings, and most of all shows his sadness about his parents' divorce and his father's absence.