While investigating when a car is pulled from Strangford Lough, DCI Tom Brannick recognises the calling card of a serial killer from decades ago known as Goliath - one of whose victims was the detectives's wife. Despite opposition from old friend DCS Jackie Twomey, Brannick and partner DS Niamh McGovern break open the cold case in the hope that it will help them.
Tom Brannick and Niamh McGovern launch a manhunt for Adam Corry who has disappeared in mysterious circumstances. Tori Matthews agrees to help Tom in the quest to identify Goliath, the legendary assassin who appears to be at large once again. As pressure builds on the police, Tom must confront the possibility Goliath may be one of his fellow officers.
Tom Brannick enlists the help of Heather Pentland to find out what Jackie Twomey knows about Goliath. Tori Matthews and Tom's daughter, Izzy, put pressure on Tom to get to the truth. With lives on the line, Tom must unmask Goliath before it's too late.