Popular crime drama Line of Duty is returning to our screens this month with more mysterious investigations as the police anti-corruption unit continue their hunt for 'H'.

After the BBC confirmed series six will start on March 21, fans think they have already figured out who the mysterious and dangerous criminal is.

Tension was also running high in the 15-second clip, which showed Superintendent Hastings saying: “You’ll be treated fairly, with the full protection of the law,” in a high stakes standoff scene.

The camera then turns to a close up shot of Kate, played by Vicky McClure, and fans took to Twitter to say that Kate is looking particularly 'guilty' in the clip.

Could Kate Fleming be the mysterious 'H'?

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One wrote on Twitter: "Kate Fleming, H?”

Whilst another added: “I think that. Undercover all the time.”

A third agreed: “Kate looking very guilty in this preview clip.”

Vicky McClure, Adrian Dunbar and Martin Compston will return, while the show also sees Shalom Brune-Franklin play a new addition to the AC-12 team.

And Kelly Macdonald makes her debut as guest lead Detective Chief Inspector Joanne Davidson, the senior investigating officer on an unsolved murder case whose suspicious conduct attracts the attention of Anti-Corruption.

What happened in season 2?

*Spoilers below

In season five, we see Tommy revealed as the witness was killed in a police ambush.

DI Denton is behind bars, but will Dot's corrupt ways finally come to light?

DI Dention (Keely Hawes) is the only surviving individual from the attack and is immediately placed under suspicion.

The truth is, Dot arranged the hit, in order to protect his secret identity.

Denton, who was part of the plan but never knew the identity of the mastermind behind it, takes the fall and is imprisoned for life.

What is going to happen in season 3?

The plot for the next instalment isn't yet clear, but the last series left viewers on the edge of their seats as the controversial police anti-corruption unit continued their hunt for the mysterious and dangerous 'H'.

Adrian Dunbar, Vicky McClure and Martin Compston all reunited in September to reprise their roles as Superintendent Ted Hastings, Detective Inspector Kate Fleming and Detective Sergeant Steve Arnott after a delay in filming due to the coronavirus pandemic.

Line Of Duty debuts at 9pm on Sunday, March 21, on BBC One, and will air weekly on BBC One and BBC iPlayer.