The Case of the Duplicate Daughter: Erle Gardner

Muriell Gilman left her father at the breakfast table while she cooked seconds of sausage and eggs. When she returned, he had disappeared.
She searched the house from cellar to attic. Then she went out to the workshop . . . there, scattered on the floor, were hundred-dollar bills, an overturned chair, and a spreading, crimson stain. That's when she telephoned Perry Mason.

Perry Mason has so many questions: why did she call him? Why didn't she want her step-sister to talk to him? And why was Gillman's wife being blackmailed - by a female private investigator . . . ?