I.D. is a 1994 British film made by BBC Films about football hooliganism,
directed by Philip Davis and starring Reece Dinsdale, Sean Pertwee and Warren Clarke.
It is set in England, mainly London, as well as Rotherham and Bradford in the 1980s.
The tagline is "When you go undercover, remember one thing... Who you are."

An undercover cop finds that the line between his own personality
and that of the character he's created have begun to dangerously
blur in this drama. John (Reece Dinsdale) is a British police detective
whose skill is matched only by his arrogance. Acts of brutal
hooliganism have become commonplace at the football matches featuring one of London's
minor league teams, Shadwell Town, and the police suspect that there is a more criminal
undercurrent to these actions than merely fandom gone wrong
after a few pints of beer. So John is made part of an undercover
team along with Trevor (Richard Graham) and two other officers;
they are to blend in with the most rabid fans and learn what is
behind the violence. John also makes the acquaintance of Lydia
(Saskia Reeves), a barmaid at a pub where many of the hooligans hang out, he and
becomes friendly with her as a way of obtaining more information. But as John sinks deeper
into a life of alcohol and violence while hanging out with the Shadwell Town hooligans,
he finds he likes it more and more, and in time, he finds that he's becoming one of the
brutal thugs he set out to capture. He also finds his relationship with Lydia is no longer
just a matter of business, much to the chagrin of his wife Marie (Claire Skinner). I.D.
marked the feature debut for British director Philip Davis, who also directed several
distinguished productions for U.K. television. 
 
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http://www.imdb.com/title/tt0113375/