The Franchise Affair — Josephine Tey

She had been kidnapped, beaten, and held prisoner the demure young girl claimed. And she denounced two highly respectable (if somewhat eccentric) women as her captors. Her story was damning: the attic room was so accurately described and there were the cracked window, the kitchen, the old trunks. How could she know everything about the house if she hadn't been held there?

But Robert Blair, lawyer for the accused women, didn't believe the girl's story. And, in a stunning piece of amateur detecting, he discovered the amazing truth even Inspector Grant hadn't conceived of!