1923. Mary Russell Holmes and her husband, the retired Sherlock Holmes, are enjoying summer on their Sussex estate when they are visited by Miss Dorothy Ruskin, an archeologist just returned from Palestine. She leaves in their protection an ancient manuscript which hints that Mary Magdalene was an apostle—an artifact certain to stir up a storm in the Christian establishment. When Ruskin is suddenly killed in a tragic accident, Russell and Holmes find themselves on the trail of a fiendishly clever murderer.