A dismembered body is discovered, and worries grow when DCI Roz Huntley fails to report for duty. AC-12 crank up their investigation into Operation Trapdoor, concerned that Michael Farmer will go to jail for crimes he didn't commit. With suspicions that Roz may have withheld crucial evidence, a new DCI is drafted in to run Operation Trapdoor.
Kate makes progress in her undercover role, but AC-12's investigation falters when they realise that their main informant, forensic investigator Tim Ifield, has also gone missing.
Steve Arnott is a young officer who's fallen foul of his superiors for refusing to help in the cover-up of an operation that ended in the shooting of an innocent father. He seems ideal to join AC-12, an anti-corruption police unit, just as it starts to investigate Detective Chief Inspector Tony Gates, the regional force's Officer of the Year.
Line of Duty - Series 4, Episode 2 Synopsis and Schedule