Many viewers believed he would be unveiled as "the fourth man".
There was a blink-and-you'll-miss-it moment going back as far as series one.
The drama's creator did not hold back after one social media user branded Line Of Duty a "cheap little show".
The police drama's conclusion has sparked a lot of conversation on social media.
Line Of Duty’s First Series Had A Major Clue About The Identity Of The Fourth Man, But Did You Spot It?
Writer Jed Mercurio had planted a hint in the 2012 series but it passed the show’s armchair detectives by.
The Steve Arnott actor said he “fully understood” that the conclusion “wasn’t the 'urgent exit’ type ending some anticipated”.
We've uncovered all the secrets of AC-12 – because Hastings and co aren't the only detectives round here.
We should both applaud the show for casting a character with learning disabilities, and call out its throwaway lines perpetuating sad old stereotypes, writes former police officer Jim Gamble.
Here's something to whet your appetite ahead of the new season's debut on Sunday.
Kelly is playing the enigmatic DCI Joanne Davidson in the sixth series of Jed Mercurio's hit BBC drama.