In different parts of London, three recruits prepare for their first day at the Metropolitan Police's training centre at Hendon. All three had succeeded in getting into the police in spite of weaknesses. But on their first day, the assistant commissioner announces that he wants them to join a team of undercover detectives. Their brief? To become criminals; to work their way up through whatever criminal organisations they can get access to, and to collate evidence against the criminals they come across. Their target? One of the world's biggest drug dealers, Den Donovan, alias 'Tango One' - number one on HM Customs and Excise List of most wanted criminals. Three years later all the recruits are getting close to their target. Too close, perhaps, to remember the rules...