A woman is found dead in Poole Harbour, throwing DCI Lesley Clarke and the two teams she's now managing at Dorset Police HQ into a complicated case.

Initially, the team believe the victim to be Jackie Kendall, recently reported as missing; an open and shut case. But when forensics reveal it to be Rowena Sharp, who mysteriously vanished fifty years ago after leaving her baby at a Sandbanks hotel, it seems that the squad have two cases, and multiple riddles to solve.

And when Jackie turns up in Cumbria saying she's witnessed another murder, it’s clear there are dark secrets to unearth. What exactly was it she saw, and does somebody want to kill her? And are the two deaths, separated by decades, connected?

Something dark lies below the waters of the harbour and it’s up to Lesley and her team to crack the case.