The theme of a story is its location
its key plot points
its unifying idea
its characters

Its unifying idea

Explanation:

The theme of a story is usually just one word. It is what the whole point of the story is. It could just be "friends" or "work hard." The theme is not the location of the story. It isn't literal. It is not the key plot points. The key plot points(the main things that happen in the story) do bring out the theme in a story, but they are not the theme. The theme is not the characters, but the writer of the story usually uses the characters to bring out the theme also.

For example, here is a made up story:

Mathew wanted a sandwich. He stood with his face pressed against the glass of the sandwich shop. He was not too hungry. He was just a little hungry. Mathew did not have enough money to buy a sandwich. He walked home and decided to take a nap instead, as he realized that he should be happy with what he has.

The theme for this story would be gratefulness. You should be grateful rather than always want more. The character, Mathew, helps bring out the theme by wanting a sandwich but instead going to bed. The locations are the sandwich shop and his home. These help bring out the theme by showing that he should be grateful that he has a home rather than want a sandwich. The key plot point is Mathew wanting a sandwich. This helps bring out the theme by showing that Mathew was ungrateful at the beginning.

The theme is the unifying idea. The writer uses plot points, characters, and locations to give the reader a lesson.