Fans are hotly anticipating the episode, which will see the assailant finally unmasked, and hopefully reveal the connection he has to corrupt officer DCI Roz Huntley.
But speaking to the Evening Standard, Martin, who plays AC-12’s DS Steve Arnott, has claimed viewers may have to cast their minds back to previous series to crack the case.
Describing the killer’s identity as “a bit of genius”, he said: “Every series stands alone. But I think it would have helped if you’d watched the first couple.
Adrian Dunbar, who plays AC-12 chief Ted Hastings, previously teased fans with a theory about the dubious ACC Hilton.
Asked about the conflict between police chiefs Hastings and Hilton, Adrian told TV & Satellite Week magazine: “What Hilton is a part of hasn’t been entirely resolved and we aren’t sure if Ted knows more about that. Those things may turn out to be red herrings, though.”
A trailer for the series finale revealed there will be a final shoot-out at the head-quarters of AC-12, but it remains unclear who they are standing off against.
The net is also seen closing in on DCI Roz Huntley, who has framed her husband Nick for the murder of Tim Ifield, as AC-12 continue to try and get to the bottom of her deception.
Elsewhere, the forensic team is also seen on the scene of a fresh discovery - could Balaclava Man have struck again?
‘Line Of Duty’ concludes on Sunday at 9pm on BBC One. You can catch up on the series so far on BBC iPlayer.