Why Didn't They Ask Evans?

When Bobby Jones and Lady Frances "Frankie" Derwent play a round of golf, Bobby slices his ball over the edge of a cliff. His ball is lost, but on the rocks below he finds the crumpled body of a dying man. With his final breath the man opens his eyes and gasps, ‘Why didn't they ask Evans?'  Armed only with the image of a beautiful young woman found in the dead man's pocket, the amateur sleuths must solve the case.