It's the breezy Scottish summer of 1936 and aristocratic sleuth Dandy Gilver, along with trusted colleague Alec Osborne, has been called to solve the strange case of the Cramond Ferrywoman, on the Firth of Forth. From their cheerless digs in a local stately home, Dandy and Alec track Vesper Kemp, the ferrywoman, to a tiny tidal island. She seems to have lost her mind, roaming the beaches in rags, ranting about snakes and mercury. What is even more troubling, is that Vesper claims she murdered Peter Haslett, a young man who fell into the river, trying to row past one of its four water mills, and drowned. A group of worried Cramond residents - the minister, the innkeeper and the lady of the big house - are determined that Vesper is innocent.