Some Line of Duty viewers now believe they've cracked the case, and know the possible secret link between two characters.
Amid speculation that Marcus Thurwell is still alive, after being confirmed dead recently, fans think he shares a "missing link" to Steph Corbett.
The widow of undercover officer John Corbett has remained in the series following his murder, and fans are predicting she has a huge role to play.
With AC-12 and viewers at home finally closing in on the mystery identity of 'H', many are wondering if it could be Thurwell, despite his body apparently being found at his Spanish home.
A video which aired in the latest episode featured a masked man who appeared to have similar eyebrows to Thurwell actor James Nesbitt, and it's got heads spinning.
Now, many are predicting he will return in the finale - and that he could be linked to Steph.
It's all because she apparently appeared shifty in recent scenes at her home, almost as though she is hiding something.
Not only that but her stint in the latest and potentially final series of the BBC crime drama has been pretty quiet, so viewers are wondering if there is more to her being back.
Taking to Twitter, some viewers began to speculate that she could be hiding the real Marcus Thurwell in her home and that this is their secret link - all to possibly be exposed in the big finale.
One viewer wrote: "I predict that Marcus Thurwell is living with Steph Corbett. The bodies in Spain were from the freezer."
Another agreed: ""Marcus Thurwell is staying with Steph Corbett and the money in the attic is his."
A third simply tweeted: "Marcus Thurwell and Steph."
Others were just convinced that Steph was up to something, with the truth behind the £50,000 she was given by Ted Hastings also yet to be cleared up.
Line of Duty concludes Sunday at 9pm on BBC One.
