It is Christmas Day in York. A woman is found dead in her bed. It is an apparent suicide; a note is found on her chest. It reads 'I am so sorry Martin'. Hours later the police are called to a house a few miles away. A mother and her daughter have been brutally attacked; the mother is dead and the daughter is unconscious. The father is found shaking uncontrollably in the corner of the room. He is covered in blood but he is unharmed. His name is Martin Willow. Detective Sergeant Jason Smith wakes up on Christmas Day after a recurring dream; a dream that began after his sister disappeared in Western Australia ten years before from what appeared to be a shark attack. He is under the water and he tries to reach out to his sister but she sinks to the bottom. He is put in charge of both murder investigations. After reaching dead end after dead end he figures out that Martin Willow has been framed for the attacks and the killer was seeking revenge for an event in his past he believes Willow was responsible for. A man from Smith's past appears and tells him his sister is still alive and involved in a secret cult. This coincides with Smith being forced to take a leave of absence from the force after beating up a suspect in a jail cell. Smith starts off on a search for his sister which takes him deep into the criminal underworld in which this cult exists. He finds her only to watch her disappear again. The little girl who was attacked on Christmas Day wakes from her coma and gives Smith a clue about the attacker; a piece of a song she heard directly before the attack. Smith is then forced to face his worst fear, the fear of the ocean, as the killer kidnaps his colleague and only friend, Detective Constable Erica Whitton and holds her at gunpoint on a boat in the middle of the sea.