Mary Graham had taken Jock Hepburn as a lover and gotten pregnant. Her brother challenged Hepburn to a duel. Because she knew her brother could better Jock, Mary killed her brother by shooting him in the back. Robert Carey deduced Mary's involvement after he noticed she was pregnant and because she had a tight bandage on her right wrist.
Carey intended to arrest Mary and put her on trial for murder, but her father assigned responsibility for his son's death to Hepburn and took his revenge.