A thesis statement about the novel and the answer to how the epistolary genre functions in the novel.
The introduction paragraph will introduce and briefly summarize the novel. It should also briefly mention the epistolary form/genre and transition to your thesis statement(most usually places at the end of the introduction paragraph).
The body paragraphs will discuss/explain your thesis statement, using evidence from the novel.
How the novel/plot/theme would be different if it were not written in the epistolary form.
The conclusion paragraph will go beyond summerizing your main points and leave the reader with a great sense of the greater importance or significance of story telling in this (the epistolary way) and the effect on the reader.
Sample Solution