The BAFTA-winning hit series returns, as DCI Cassie Stuart (Nicola Walker) and DI Sunny Khan (Sanjeev Bhaskar) investigate a cold case with alarming links to the police force. A corpse is discovered in a scrap yard, his features frozen in time. Four suspects emerge. All four have links to the law and attended the same police training course back in 1989. Since then, their lives have gone in very different directions. Some have thrived and others have faltered. But exactly who is fighting for justice? Can the police force ever really be trusted? And will Cassie and Sunny find themselves on the wrong side of the law?