Danny Waldron (Danny Mays), the highly proficient leader of a police Armed Response Unit whose unpredictable behaviour is becoming a threat to colleagues and suspects alike. When AC-12 are called in, DS Steve Arnott (Martin Compston) clashes with Danny, and it’s left to DC Kate Fleming (Vicky McClure) to investigate undercover. As the inquiry intensifies, Supt. Ted Hastings (Adrian Dunbar) begins to suspect the case is linked to AC-12’s past and a possible traitor in their midst.