David Dexter and Ira Thatcher are returning from embassy duty in Europe and meet in Bermuda, where they report on the state of naval preparedness in England and France, especially regarding ironclad ships. The Confederate Navy is converting the Merrimac into a submersible armored giant and in response the U.S. Navy is building the Monitor.
David joins the Confederate Navy as a spy, boards the Merrimac, and serves in battle against best friend Ira on the Monitor.