If you're specifically looking for a flashback story within the "Keeper of the Lost Cities" series, it might be integrated into one of the books. Shannon Messenger's series includes several novels and companion books, such as:
You can find a collection of downloadable guides for each book in the series on the Shannon Messenger Educators Page . Flashback: Quick Overview Educators - Shannon Messenger keeper of the lost cities flashback pdf link
Unfortunately, I couldn't find a legitimate PDF link for a flashback review of "Keeper of the Lost Cities." However, I can suggest some alternatives: If you're specifically looking for a flashback story