The plot is what happens in the book, the theme is the underlying idea/s being explored in the book.
If you are anything like me, you will find yourself coming back to explore certain themes because they are hard wired into your psyche.
In Dickens’ books there is the underlying theme of social injustice. In Joanna Russ’s books there is an underlying theme of gender.
Are you aware of your theme as you are writing? I often discover the theme after I’ve finished the book. Then I go back and use my awareness of the theme to make scenes more powerful. (Without hitting the reader over the head with the theme).
Over at the KRK blog I’ve done a post about how writers tell lies (stories) to reveal inner truths (the human condition). These are the themes writers explore.
Before trying to find the theme of the stories and books you are currently writing, take the time to think about themes in a larger sense. Write a list of your ten favourite books and movies. Now what is the theme of each one? Are these themes related? Is there a pattern to the type of book/movie that draws you in?
Now that you’ve analysed why those books and movies drew you and you are aware of their themes, think about the books and stories that you have written over the years. Every story and book requires an emotional investment from you. What were the themes that you explored? Can you see a pattern?
Are the themes you write about different from the themes you find yourself drawn to in movies and books?
Now that you are more aware of theme, take a look at your most recent short story. Now that you know your theme, you can use this insight to make the ending more powerful by layering subtext into the narrative.
And, just for fun:
Theme – a powerful tool.