In the first of a new series, Allan Urry investigates claims by former officers from one of Britain's biggest police forces that they've been the victims of crimes committed by their own colleagues. He hears claims of dirty tricks by a secretive police unit within Greater Manchester Police which some officers say have led to criminal charges against them. Others say they've been unfairly targeted through the internal disciplinary process, with evidence distorted and statements changed. Are they bad cops with an axe to grind or victims of corrupt practices and institutional cover up? Producers: Sally Chesworth and Neil Morrow.