Narrative Through-line, Narrative train, narrative spine

I recently sent my documentary short to be reviewed by my professor that taught the class documentary storytelling using Sheila Curran Bernard's book.

He mentioned that he was very proud of my accomplishments. He also pointed out that he didn't understand what the narrative through-line was.

I let his email sink in for a few days realizing I needed to go back and read Bernard's book and research the internet on this topic. The narrative through-line, the narrative train, the narrative spine that holds the story together. After all my research, I realized that Bernard's book nor any of the information I researched on the internet explained how to look for the through-line of a subject. Nor did I find how to identify the narrative spine.

Does anyone have any insight or can reference any websites to help me develop this part of my documentary?

If you would like to watch my short, it can be viewed at http://www.cabslaves.com

Thanks,