Millions of Brits are enjoying the return of BBC hit Line Of Duty as episode three of series six airs tonight at 9pm.

So much so, the first episode of the latest series broke records when it became the most watched episode of drama since 2018.

Consolidated figures from audience research organisation Barb showed a total of 13.1 million viewers watched the programme, which returned to television screens last month.

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It was the most watched episode of drama since the finale of Bodyguard, also created by Jed Mercurio, in September 2018, which had an audience of 14.3 million.

The new series of the police corruption show sees Vicky McClure, Adrian Dunbar, and Martin Compston reprising their roles, while Kelly Macdonald has joined the cast.

The show returned on March 21, with anti-corruption unit AC-12 investigating a new case regarding the senior officer in an unsolved murder case.

But like all shows created by screenwriter Jed Mercurio, Line Of Duty is not the easiest to follow at times, always keeping the audience guessing.

Not least with the endless list of acronyms that make following the story line almost impossible for the uninitiated.

Season six newcomer, Kelly Macdonald admitted she struggled to get her head around the script when she first took on the role of DCI Jo Davidson.

She said: “There would be certain acronyms that we would discuss at the time.

“In hair and makeup, they had a list pinned up, which was quite helpful, I found, of all the acronyms and what everything stood for. I’m not brilliant at [remembering them], but I sort of get by.”

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So, to keep on top AC-12’s antics, you will need to know your CHIS from your AFO.

To help you along, here is a Line Of Duty cheat sheet as the nation prepares for another episode of the BBC crime drama.

Line of Duty acronyms

  • CHIS - Covert Human Intelligence Source
  • MIT - Murder Investigation Team
  • PNC - Police National Computer
  • TA - Tactical Advisor
  • AC-12 – Anti-Corruption Unit 12
  • ARU - Armed Response Unit
  • ARV - Armed Response Vehicle
  • AFO - Authorised Firearms Officer
  • TFC - Tactical Firearms Commander
  • AM - Active Message
  • Sit Rep - Situation Report
  • SIO - Senior Investigating Officer
  • DIR - Digital Interview Recorder
  • UCO - Undercover Officer.
  • B&E - Breaking and entering
  • CID - Criminal Investigation Department
  • CIS - Crime Information System
  • CPS - Crown Prosecution Service
  • MIT - Major incident team
  • NCS - National Crime Squad
  • Fahrenheit - AC-12's codeword for 'shoot to kill' - this is different for each police operation.

Police rankings

  • DC - Detective Constable (cops who have passed their detective exams get a “D” instead of “P” before their rank)
  • DS - Detective Sergeant
  • DI - Detective Inspector
  • DCI - Detective Chief Inspector
  • Det Supt - Detective Superintendent
  • DCS - Detective Chief Superintendent
  • ACC - Assistant Chief Constable
  • DCC - Detective Chief Constable
  • CC - Chief Constable