This book starts out as a thriller, and pretty soon it becomes clear that there are elements of the spy novel as well. But I think it belongs more in the tradition of the gothic novel, what with seances, an old house with a reputation of being haunted, terrifying servants...
The story takes place during WWII. Sarah Marlowe is forced to wait for her train connection, and can't help entering into a conversation with a fellow-traveler, a mousy woman called Emily Case, who spins a confused tale of having been entrusted with a package by a young man who was stabbed. Sarah dismisses the story as the ramblings of a neurotic spinster... until she finds a package in her handbag. The package contains list of addresses and a picture of a man. When she hears that Emily Case was found murdered, she debates whether to respond to the police's request for all witnesses to come forward. She decides against it, because she feels that her employer, Wilson Cattermole, would not like it and would perhaps even terminate her employment. Sarah's role as Wilson Cattermole's secretary is to take take notes of his experiments in psychic research, and to keep an eye on his scatterbrained sister Joanna. Then Wilson decides to have the chauffeur John Wickham drive himself, Joanna and Sarah to an old country mansion, supposedly to conduct psychic research with his friend Mr. Brown. Soon it becomes clear that Wilson Cattermole and Mr. Brown are out to scare Sarah into handing over Emily's package. But what role does John Wickham play? Is he in with the bad guys or is he on Sarah's side? Things come to a head during a dramatic night that includes a faked seance, an imprisonment, a car accident and attempted murder.
The book is interesting because it offers an insight into the themes that preoccupied the British reading public in 1941 (the year this book was published). The fear that diabolically clever German spies were creating a Fifth Column, of course, but also the deprivations of wartime, a preoccupation with supernatural phenomena, and the increasing need for women to make a living.
The book has a couple of obvious weaknesses, such as the fact that Sarah's admirer Henry is given a prominent role in the first part of the book and seems to have been forgotten in the second half. John Wickham is introduced into the story in a rather clunky way. The plot twist of Sarah having found a safe spot to put the papers, while allowing the crooks to chase a decoy, is dealt with in an unnecessarily coy fashion.
In short : a novel that is one third gothic novel, one third spy thriller, and one third romance.