Shocked by the extent of corruption within its ranks, Scotland Yard set up a new anti-corruption unit in the early 1990s. Its members had to operate in conditions of unprecedented secrecy and they became known as the 'Ghost Squad'. Bent coppers really did believe they were untouchable: they stole cash and property, fitted-up innocent people and sold secret information to cripple court cases. Many of the bent coppers are now in jail or awaiting trial but the battle against corruption is not over. Only now can the story of the 'Ghost Squad' be revealed. Award-winning BBC home affairs correspondent Graeme McLagan has followed the investigations since the beginning. He has interviewed undercover officers and many of the bent coppers they have exposed. This is the inside story of the 'Ghost Squad' and how it broke into the secret world of police corruption.