Fans of the police procedural drama will be eager to find out if Nelson-born creator Jed Mercurio finally reveals the identity of corrupt police officer H.
But with a sixth series confirmed by BBC One, the showrunner could keep viewers guessing past the finale.
The show follows the work of anti-corruption unit AC-12, led by Superintendent Ted Hastings, DS Steve Arnott (Martin Compston) and DI Kate Fleming (Vicky McClure).
Series five saw a number of characters meet a grisly end, including the unit's very own Pc Maneet Bindra (Maya Sondhi) in episode one.
Stephen Graham's character John Corbett was killed off in episode four in shocking scenes which saw the undercover police officer have his throat slit by another member of the organised crime group.
Viewers were left reeling last week after Hastings (Adrian Dunbar) was arrested under suspicion of being a corrupt police officer.
In tense scenes, the long-running character was quizzed in the now infamous glass-walled office by Anna Maxwell Martin as Superintendent Patricia Carmichael.
The finale episode of Line of Duty airs at 9pm on BBC One.