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 ones 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. But with four local millers themselves remaining oddly tight-lipped and with all the suspicious strangers who lurk about the village, Dandy and Alec have their work cut out for them. And the closer they get to the answers they seek, the stronger the sense that great danger lies beneath the surface of these murky waters.