The BBC has announced that season five guest star Anna Maxwell Martin will return to Line Of Duty on Sunday.
The actress will reprise her role as Detective Chief Superintendent Patricia Carmichael, the corporation said.
Martin's Carmichael appeared in season five of the acclaimed cop drama, when she was brought in as an independent investigator amid doubts over the integrity of Adrian Dunbar's Ted Hastings.
Her return comes as Ted is facing a forced retirement, potentially opening the door for Carmichael to take control.
Martin said: "Pat's back, I can't remember what she's doing there, nor would I be allowed to blab, but suffice to say she's got some new clobber and she's ready to bust some balls."
Martin, 43, is a two-time Bafta TV Award winner. Her previous roles include the BBC's 2005 adaptation of Bleak House, the 2015 Agatha Christie adaptation And Then There Were None, and comedy series Motherland.