The two men look at each other in alarm. The eerie tapping at the window is repeated. "Someone has seen us! Run!" one of the burglars exclaims. In an instant the light goes out and two dark figures run out of the house past the girls concealed in the shrubbery.

Kay Tracey's quick-witted action in tapping on the window pane has foiled the robbery at the home of the queer old Crowley sisters. But the mysterious adventures of the teen-age detective and her fiends Betty and Wilma Worth are only beginning. What is so valuable in the old Crowley home that burglars would single it out for attention?

Searching for clues over the desolate sand dunes, Brantwood's amateur sleuth finds more problems than she has bargained for in the tangled lives of poor Ben Wheeler and a missing young couple. And adding to the excitement is Kay's coolheaded rescue of a famous airplane pilot downed at sea.

Kay's relentless search for the key to the mysterious happenings finally results in the breathtaking and almost unbelievable solution.