The advertisement in The Times -
“Glenvarroch. Scottish family with large house on West Highland sea loch welcomes paying guests for summer holidays”
- attracted an oddly assorted group: a schoolboy, an American couple, a schoolmistress from the Midlands, a young man writing a novel and an exotic brother and sister from London. But after a while the charm of the visitors begins to pall, and the C.I.D. warn Professor McKechnie that his daughter Fiona may be in danger.