Line of Duty returns for a second series, featuring a new police corruption story told over six one-hour episodes. The series opens explosively with the ambush of a police convoy in which three police officers are killed and a protected witness seriously injured. The sole surviving police officer is Detective Inspector Lindsay Denton (Keeley Hawes). Deputy Chief Constable Mike Dryden (Mark Bonnar) takes personal charge, assigning AC-12 to the case.

Detective Sergeant Steve Arnott (Martin Compston), Detective Constable Kate Fleming (Vicky McClure) and Superintendent Ted Hastings (Adrian Dunbar) are joined by newly-appointed Detective Constable Georgia Trotman (Jessica Raine) to investigate a complex case rich in unexpected twists and turns.