Lawyer and former Union soldier Richard Montgomery is commissioned to infiltrate a clandestine Confederate operation in Canada in 1864 during the U.S. Civil War. On numerous forays across enemy lines as a courier for former Alabama senator Clement C. Clay, one of the leaders of the Rebel conspiracy, Montgomery relays the word to his Yankee superiors regarding several diabolical Confederate campaigns in Union territory. These include a nefarious attempt to burn down much of New York City, guerrilla raids into New England, and ultimately the assassination of President Abraham Lincoln, the last involving early planning in Montreal with Confederate conspirators such as John Wilkes Booth.

Posing as a Southern sympathizer named James Thomson in St. Catharines, Canada West, a hotbed of Rebel intrigue, Montgomery reunites with childhood friend Aaron Young, a once enslaved man who escaped the plantation of Montgomery's father and becomes an ally in the secret Union mission. To complicate his life further, Montgomery meets a Southern woman living in St. Catharines and falls in love.

Along the way, he also encounters famed Montreal photographer William Notman, as well as various spies and secret agents on both sides of the war in Toronto, Niagara Falls, Montreal, and Halifax, eventually crossing paths with Lincoln's future assassin.