The show centers around members of AC-12, a division of the Anti-Corruption squad, so they are called in where there's an officer-involved shooting or any sort of perceived impropriety. DS Steve Arnott (Martin Compton) works closely with DC Kate Fleming (Vicky McClure), while Matthew "Dot" Cottan (Craig Parkinson) is a more recent addition to AC-12 this season, although he was present in previous seasons, just not in AC-12. Lastly, the team is headed by stalwart Superintendent Ted Hastings (Adrian Dunbar), with AC-12 staff lawyer Gill (Polly Walker, Prisoners' Wives) occasionally poking her nose into things.
Line of Duty: Series 3 kicks off when several teams of AFOs (Authorized Firearms Officers) are sent to stop a gangland murder said to be happening that day. One team, led by AFO Danny Waldron (Daniel Mays), stops the vehicle the group is pursuing and Waldron gives chase on foot, ending in the suspect's shooting death. Since the events surrounding the shooting are highly suspicious, the AC-12 is brought in to investigate and Kate Fleming is actually sent in undercover as a new member of Danny's team to suss out what really happened.
Meanwhile, Lindsey Denton (Keeley Hawes, voice of Lara Croft in Tomb Raider, The Murdoch Movies) has been granted an appeal trial based on the happenings of last season, and she is released based partly on an unprofessional relationship she had with Steve Arnott. This leaves Denton free to figure out who framed her, and her case ties back into everything happening with Danny Waldron.
When tragedy strikes Waldron's team while out on a call, the members of AC-12 begin to peel back the layers of what Waldron had begun to investigate and a mysterious bent copper called "The Caddy" once again rears his head and begins pulling the puppet strings of certain cops to his own benefit. Before long, circumstances make it appear as though Arnott is crooked and even Fleming begins to believe it, based on information Cottan is providing to her. As a child sex ring is revealed, whose members have powerful positions, it will become unclear just who can and can't be trusted in AC-12. The whole shebang comes to an explosive end in the final episode, bringing about the shocking deaths of several main characters.
As I said before, I haven't seen the previous episodes and thought the series was more of an anthology. While the focus of AC-12 changes each series, there are ties to previous seasons and it is best watched in chronological order to get the most bang out of it. That said, the characters, actors, and storylines are great and really well written, so even coming in not having seen the previous seasons didn't stop me from really enjoying this show. If you like good cop drama, you should definitely check out Line of Duty: Series 3.