Series 1 originally aired on BBC Two between 26 June 2012 and 24 July 2012.
- Lennie James as DCI Anthony Gates
- Martin Compston as DS Steve Arnott
- Vicky McClure as DC Kate Fleming
- Gina McKee as Jacqueline 'Jackie' Laverty
- Adrian Dunbar as Supt. Edward 'Ted' Hastings
- Craig Parkinson as DS Matthew 'Dot' Cottan
- Kate Ashfield as Julie 'Jools' Gates
- Paul Higgins as CS Derek Hilton
- Neil Morrissey as DC Nigel 'Nige' Morton
- Nigel Boyle as DI Ian Buckells
Series one revolves around DCI Tony Gates, DS Matthew Cottan (Dot), DC Nigel Morton, DC Kate Fleming and DC Deepak Kapoor who are all part of the TO-20 squad. AC-12 led by Superintendent Ted Hastings, with DS Steve Arnott begin investigating DCI Gates about his inflated crime statistics and possible laddering. Gates's relationship with Jackie Laverty brings about untold difficulties.
However, the case was much more complex than the “laddering” (a process in which a number of different charges are placed on the same defendant to increase an officer’s number of successful cases) they suspected him of.
Gates was having an affair with a woman called Jackie Laverty, and after she claimed to have hit a dog while drink-driving, he agreed to cover it up for her by staging a break-in at her home and making it look like her car was stolen. However, he was horrified to later discover Laverty hadn’t actually hit a dog, but had killed a man during the “accident”.
In reality, she had killed her accountant after he discovered she had been laundering money for gangster Tommy Hunter, and had threatened to tell the police.
At the instruction of Hunter, Laverty was later murdered after masked raiders stormed her house during a passionate encounter with Gates. Hunter then framed an unconscious Gates by putting his fingerprints on the murder weapon, used this fabricated evidence to blackmail Gates into helping him.
After a tip-off from an undercover Fleming, AC-12 arrived at Laverty’s house, only to find Gates at the house claiming he’d come to arrest her, and had discovered her dead.
DS Steve Arnott grew increasingly determined to nail Gates, convinced he was involved in Laverty’s money laundering, and soon discovered she had used some of the money to pay his kids’ school fees.
With Arnott hot on his tail, Gates was backed into a corner and agreed to help AC-12 track Hunter down.
After finally catching up with him, Hunter admitted to Laverty’s murder and the killing of three drug dealers, claiming to be operating under the instruction of an inside man. A conversation between Hunter and one of Gates’ officers, DS Matthew ‘Dot’ Cottan, revealed that he was the inside man, known as ‘The Caddy’. He and Tommy went back a long way, as Cottan had carried Hunter’s golf bags as a kid, hence his codename.
However, Cottan’s true identity remained a secret to his police colleagues, and he continues to operate from the inside.
Realising his career was over, the series ended with Gates taking his own life, instructing AC-12 to tell his family he was killed “in the line of duty”, thus earning the series its name.
Tommy Hunter was then placed under witness protection, setting the series up for its next chapter...