Episode four will air in the UK on BBC One later tonight (11 April).
On Twitter, Compston, who plays Steve Arnott, described the episode as “the big one”.
He wrote: “On paper one of the best @jed_mercurio has ever written, if we deliver what was there potential to be a classic.
“Like to give a shout out to the young team Shalom and @gregorypiper98 who give outstanding performances in the midst of the madness.”
Shalom Brune-Franklin plays DC Chloe Bishop, who works alongside Arnott in AC-12, while Gregory Piper plays Ryan Pilkington, the crooked police rookie with ties to the Organised Crime Group (OCG).
You can find a breakdown of what happened in last week’s episode here.
Since the airing of episode three, a number of fans have spotted a clue which possibly links to the identity of “the fourth man” (formerly known as “H”), hidden in a kitchen tile in the background of a scene.
Some viewers also noticed a filming blunder during one scene featuring Compston, prompting a humorous response from the show’s official social media account.
Line of Duty continues on Sundays at 9pm on BBC One.