Man Who Fell Through the Earth

A lawyer is leaving his office on the top floor of an office building. He sees the shadows of two men fighting through the clouded glass of an office door followed by a shot from the office across the hall. He goes to investigate. He finds no sign of either victim or assailant despite the fact that no one could have passed him in the hallway without being seen. A murder has been committed, that of the banker. Who is the murderer? A business associate, the banker's beautiful ward, or a mysterious woman who had been in the office earlier? And what part, if any, was played by the amnesia victim pulled from the river; a man who insists that his earliest memory is of falling through a hole in the earth?