Since Gettysburg, the Civil war was going well for the Union, with successes throughout the south and west. Abraham Lincoln still feared the inventiveness of Robert E. Lee. Lee, in fact, had developed a plan to get an army past the Union blockade of the Chesapeake Bay and attack Washington and Baltimore, thereby changing the outcome of the Civil War.
Lincoln makes use of the connections of his sometimes ally, sometimes adversary, Frederick Douglass to arrange a partnership with Chesapeake Bay Pilot Zachariah Clark to thwart what they thought was Lee's potential plan. Douglass and Clark had lived near one another near the Baltimore waterfront where they both worked.
Each side in the conflict was forced to develop its own spy network and espionage strategies almost from scratch. Both sides needed new strategies and tactics, since nearly all the officers on both sides had received the same training at one of the two service academies at West Point and Annapolis. Almost none of the officers and ordinary soldiers had any experience fighting a war against an opponent who was well-equipped and well-trained. Each side had to devise new ways to attack the other. Spies were everywhere.
Lurking in the background is John Wilkes Booth who devised three separate plots, one of them involving international partners, to remove President Lincoln from office either by kidnapping or by assassination.