The Broken Horseshoe plot
Charles Constance is the victim of a traffic accident. dr. Mark Fenton saves him in the nick of time. The perpetrator drove on. A young woman, Della Freeman, delivers flowers in the shape of a broken horseshoe, but Charles claims not to know her. Before leaving the hospital, Charles hands doctor Mark Fenton an envelope with the request to post it if anything should happen to him. And indeed, a week later the doctor finds him murdered. The mysterious lady reappears, and she persuades the doctor not to tell the police anything about her. Thus Mark Fenton ends up in a vortex of murder and deception, with the contents of the envelope as the bet.