The Shadow on the Door — Frances K. Judd

"I can't put my name to this." Kay is taken aback when Mrs. Van Hutton refuses to sign a paper prepared by her lawyer, Bill Tracey. When Kay notices that Mrs. Van Hutton has a secret meeting with a man named Stannovi, Kay suspects that Stannovi is influencing Mrs. Van Hutton in her decisions. Complicating matters, Stannovi is engaged to Mrs. Van Hutton's secretary, Ellen Tremont, who has complete faith in Stannovi. Kay tries to warn Ellen with no success.

Later, Kay receives a note warning her to stay away from Mrs. Van Hutton and out of her affairs. Who sent Kay the warning note? And what is that person afraid that Kay will discover? Kay continues her investigation, which is hindered by the antics of Ethel Eaton, her rival.