The Fourth Horseman tells the story of the 20th century's first foray into biological warfare, a World War I German Army sabotage campaign that featured a "germ factory" in the basement of a cottage in Washington, D.C. The book's main character is a Virginia-born doctor and German spy, Anton Dilger, who studied medicine at the University of Heidelberg and Johns Hopkins University, and was the descendant of a great German physiologist.