Bonaventure by Charlotte Hastings
A flood in the fen country has trapped two prison officials and their prisoner, Sarat Carn, who is on her way to the gallows for the murder of her brother.
In spite of the evidence, Sister Mary Bonaventure is convinced of Sarat's innocence and she uncovers an unknown connection between Sarat's friend, Dr. Jeffries. It transpires that Sarat's brother was responsible for the death of a woman.
Just before the police launch arrives, a confession is forced from the real murderer.