On December 12, 1910, a 25-year-old heiress named Dorothy Arnold went shopping on Fifth Avenue, chatted briefly with a friend, waved goodbye, and was never seen again. Her disappearance opens a window into the suffocating world of Gilded Age New York high society, where a family's standing in the Social Register was the currency they traded on.

Terrified of scandal, the wealthy Arnold family covered up Dorothy's disappearance for six weeks, hiring private Pinkerton detectives instead of calling the police and even destroying potential evidence. By the time they went public, the trail was frozen, leaving behind only competing theories, secret lovers, and a mystery that still has no answer over a century later.

• How a thousand dollars in cash and infinite store credit functioned as a straitjacket on Dorothy's independence
• Th ...