I first read this as a teenager and was a little underwhelmed: I don't think I really appreciated the rarity of good writing then or the subtle nuances of character which Tey conveys so well. Re-reading it now, I loved this book. It is, undoubtedly, of its time: published in 1949 in conveys a slightly snobbish attitude to the `lower classes' who are either demonised (Betty Kane and her mother) or sentimentalised (Stanley). But leaving that aside, this is a really fun and intriguing mystery.
Robert Blair, a staid solicitor, is drawn into a case involving the odd Sharpe women, mother and daughter, who live alone in their house named The Franchise when they are accused of kidnapping and beating a young teenager. No-one quite believes the story until the girl is brought face-to-face with the Sharpes and reveals details about the house and the room where she was supposedly held that a stranger could not know. But Blair believes the Sharpes are innocent and sets out to prove his case - against all the odds.
Like other `golden age' mystery writers (Dorothy L. Sayer, Ngaio Marsh, Christie) Tey is as interested in her characters as she is in the mystery itself, and the Sharpes, especially, are wonderful creations. Our emotions are manipulated faultlessly as they are moved from being slightly sinister to being amusingly eccentric (old Mrs Sharpe, especially, grew hugely in my affections during this book), and yet there is still always a slight doubt: could their very eccentricity have led to their guilt? Highly recommended.