While onboard her last chartered sailing trip of the season, Cass Lynch is awoken in the middle of the night by a Mayday call to the Shetland coastguard. A fishing vessel has become trapped on the rocks off the coast of one of the islands.

In the days that follow, there's both a shocking murder and a baffling death. On the surface there's no link, but when Cass becomes involved it is soon clear that her life is also in danger. Convinced that someone sinister is at work in these Shetland waters, Cass is determined to find and stop them.

But uncovering the truth could prove to be deadly ...