The story begins with a pretty young heiress and her uncle seated outside their summer home on Spanish Cape, escaping the guests at her parents' house party. Suddenly a one-eyed giant bursts onto the scene and kidnaps them, misidentifying the uncle as John Marco, a house guest. The giant removes them both, ties the heiress to a chair in an empty neighbouring home, and disappears with the uncle. Ellery Queen and a friend arrive at the neighbouring home in the morning and release the heiress, but by the time they can return her to her home, Marco has been found on the terrace, strangled, and nude except for an enveloping opera cape.
The house party and the household contain many people who had good reason to want the victim out of the way, some because he was blackmailing them. As Ellery is investigating the crime, another household member commits suicide out of sheer desperation at the potential revelation of a dread secret. By the time the uncle escapes his captor and swims to shore, Ellery is ready to reveal the identity of the murderer and the reason for the victim's nudity in a dramatic final scene.