"The Double-Cross System" claims to be an authentic document on espionage drawn up during World War II. It was written as a secret report for the Intelligence Commission and tells how Britain used the enemy against themselves. The book describes the stratagems and the agents like Snow, Zigzag and Tricycle; these individuals were known only by these names, and in them they lived and operated. It describes the methods of communication between the agent and enemy, citing examples whereby the details of a man's wireless style, the rhythm of the key in sending messages had to be mastered so that if the agent should die or be removed, a substitution could be made but not recognized.