In 1999, the BBC aired a documentary about the Chelsea Headhunters, a gang of football hooligans. Irish investigative reporter Donal MacIntyre went undercover and infiltrated the gang, wearing a concealed camera and pretending to be a Chelsea hooligan — even to the point of getting a Chelsea tattoo to convince the hooligans that he was a lifetime supporter.
The following is the video that takes you undercover. Note: Fast forward past the first two minutes to skip past the introduction in a foreign language.
After the video aired, according to Wikipedia, “In 2000, Jason Marriner, a member of the Chelsea Headhunters was sentenced to six years in prison for his part in organizing a fight with supporters of a rival team, based on evidence captured by Donal MacIntyre and his team. MacIntyre was placed under Police protection during the trial. It was the first significant victory against the hooligan fraternity since the flawed attempts at undercover work by the Police ten years previously, in the ill-fated own goal trials. MacIntyre also secured convictions against members of Combat 18.”