A narrative's main idea often is its theme: a universal statement, like "love conquers all," that is the story's focus and prime subject. It provides a comment or insight on humanity and usually is found in four attributes of the work: the title, the actions/conflicts of the characters, the narrative's pattern and the work's final impressions on the reader.
Two Clues: Title and Action/Conflict
Titles are no more than slight clues to main ideas, but examples such as "Love in the Time of Cholera," "As I Lay Dying" and "Blood Meridian" all hint at themes that are complex and dark. A second, more substantial clue to a main idea lies in the actions/conflicts of the protagonist/antagonist and his or her particular obsessions. This woman is obsessed with a proper burial for her relative, that couple is obsessed with love, this captain is after that whale. The narrative's main idea usually begins in those obsessive actions and conflicts.
Meanings in Patterns
The narrative's pattern is an even more substantial clue to meaning. Often the pattern is a character's journey seeking an answer to his obsession. What the character is seeking and what he finds frequently yield narrative meaning. Odysseus struggles to get home to his wife for all of "The Odyssey," therefore the need for home and romantic connection is the epic's main idea. Hamlet struggles not only for revenge but also for self-realization, so Shakespeare's main idea is establishing a self. In "Moby Dick," Ahab's quest kills everyone except Ishmael; the narrative's main idea is the destructive nature of obsessions.
Finishing the Work Yields The Theme
A reader usually realizes a narrative's theme when he finishes the work, since the final impressions often are the best clues to meaning. The main idea is almost always a reflection of humanity's needs or wants. Odysseus reunites with Penelope: Love conquers all. Romeo and Juliet die apart: Love is not enough in the face of society. Charles Dickens wrote two endings for his "Great Expectations," one where his hero, Pip, found love, one where he didn't, so his narrative's main idea was self-discovery, not finding love.
Reflect to Find Meanings
Sometimes the reader must reflect carefully to find narrative meaning. Ian McEwan's "Atonement," for example, pulls a thematic deception; the story seems to end well with lovers reunited, until we find out the event never happened. His narrator reveals that she has "rewritten" the ending to make it happier. Therefore, McEwan's main idea is not about love, but about wish fulfillment. Not all narrative main ideas are that difficult to unravel, but many will take some thought to successfully discover.