Daisy Darker is arriving at her grandmother's house for her eightieth birthday. It is Halloween, and Seaglass - the crumbling Cornish house perched upon its own tiny private island - is at one with the granite rocks it sits on. The Darker family haven't all been in the same place for over a decade, and when the tide comes in they'll be cut off from the rest of the world for eight hours.

When the tide goes back out, nothing will ever be the same again, because one of them is a killer.