The Confederate Ironclad.

During the Civil War, Elinor, a pretty Northern girl, comes south to visit her aunt-- Little does anyone suspect she works as a spy. Lieutenant Yancey, who's nearly engaged to the fetching and resourceful Rose, is gallant enough to show the Yankee guest around, including a walk down a hidden creek where a gunboat is built and awaits powder. Elinor sends this intelligence North, and the Bluecoats attack.