On a sweltering June evening in 1937, American Juliet Stuart Poyntz left her boardinghouse in Manhattan and walked toward Central Park, three short blocks away. She was never seen or heard from again. Seven months passed before a formal missing person's report was made, since Poyntz worked for the Soviet Secret Police and her friends (many of whom were anti-Stalinist radicals in the United States) were scared to alert authorities. Her disappearance...