When the Gaffer and his trusted duo get together in a tunnel things are usually getting very serious for the AC-12 gang - but not this time...
We may be waiting with bated breath to find how who 'H' is tonight - but Line of Duty 'best mates' Adrian Dunbar, Martin Compston and Vicky McClure have been letting their hair down, horsing around in a tunnel.
The trio appeared to be having a great time as they laughed, sang and laughed some more in the underpass much like the now famous Belfast one that often features in the hit TV show.
Dancing along, Martin and Vicky appeared to be having 'a right laugh' as they asked Adrian for a song in the subway - and what followed shows the Gaffer has talent!
In a video posted on Vicky's Instagram stories on Sunday, she says: "Adrian you are meant to sing when you're under here - it's good for your vocals" with Martin adding "sing a song".
Taking up the challenge, he and Martin starting humming 'Ahhhhhhhhhhhhhhhhhhh' with a bit of harmony thrown in - but it's tough to make out just what they were singing.
The hit coppers, however, were having a real laugh ahead of the final episode of Line of Duty on BBC tonight at 9pm.
With more than 11 million people having tuned in for last Sunday's episode, there should be a big audience for tonight's final episode of the series.
Meanwhile, Vicky has taken to Twitter to share a "happy Line of Duty Day' post as the series comes to an end.
"Belfast, our Line of Duty home. Thanks for the craic as always," she said.
"Raising my brew to you Jed Mercurio! The epitome of honesty and integrity."
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