Detective Constable Jack Mowbray investigates when the brutal murder of a young woman in Bristol sets off a chain of events, which unbeknown to him, threaten to tear his family apart and change the course of his live forever. When the Bristol murder is linked to a series of recent killings, known as the 'M4 Murders', the investigating team grows to more than a dozen detectives, and just as many petty jealousies and full-blown rivalries. Mowbray’s boss, Detective Chief Superintendent Derek Henderson, keeps the pressure on as it becomes clear that the latest killing will not be the last. With little clue or pattern to suggest the identity of the killer, the team are forced to a race against time to find the predator before he strikes again — without apparent motive. Mowbray's main sidekick is DC Jim Boulter, a no-nonsense detective who is prepared to stop at nothing to get to the truth. He often comes close to crossing the line when interrogating suspects, often forcing Mowbray or his other fellow colleagues to intervene. Other officers on the enquiry include DI Monty Fowler, a long-time close friend of Derek Henderson, who is drafted in to lead up one side of the enquiry at Henderson's request. The other senior investigating detective, DI Keith Champan was specially selected by Assistant Chief Commissioner Ronald Stocks, who has overall control of the investigation. Chapman's team also consists of WPC Margaret Walkinshaw, a rookie officer barely out of her probation period, and DCs Mickey Lloyd and Phil Leighton. Fowler's team also consists of DC Linda Harris and DC Trevor Richard. Henderson's main adversary, Detective Chief Superintendent Huw Owen, is the initial investigating officer on the case. When Owen comes close to blowing the investigation, Stocks decides to replace him with Henderson. However, Henderson later comes close to being sidelined himself, until the team identify Robert Jackson as the prime suspect for the murders. Although Jackson is initially arrested by Mowbray, Henderson makes the shock decision to release him without charge. When two further attacks occur, one of which results in a further murder, Jackson is re-arrested and eventually charged. In the beginning of the second series, Jackson is convicted of the murders, but when a subsequent attack takes place, with exactly the same modus operandi, the team suspect a copycat killer - although Fowler and Mowbray suspect that Jackson may not have been their man after all. When a civil case is brought against Mowbray and Henderson for misconduct, Mowbray discovers that vital evidence against Jackson was not discovered during the course of the initial investigation.
