It is set in 1988 in the Shadwell area of London, England and is a story about a group of Metropolitan Police officers who are sent undercover to infiltrate a gang of dangerous football hooligans.

The true events that inspired the movie are chronicled in the 2013 book Running with the Firm written by former undercover detective James Bannon.

Set in 1988, John (Reece Dinsdale), an ambitious young police officer in the Metropolitan Police, is sent undercover along with colleagues Trevor (Richard Graham), Eddie (Perry Fenwick) and Charlie (Philip Glenister) to join a violent football firm associated with Shadwell Town Football Club.

Posing as painters and decorators, John and Trevor begin frequenting the main hooligan pub, "The Rock" at lunch times and later start visiting on match days.

After becoming familiar with barmaid Linda (Saskia Reeves) and some of the regulars, they start attending matches with hooligans Martin (Sean Pertwee), Nik (Charles De'Ath) and Gumbo (Lee Ross).

Firm leader Bob (Warren Clarke) who was initially suspicious of John, now lauds him as a hero for leading the charge against the away fans.

Later, John pretends to be unable to read when playing on a pub quiz machine which appears to get rid of any certainty that he is a policeman.

During a stand off inside the stadium the home fans begin throwing objects, one of which a dart hits and injures Gumbo.

It is later revealed that one of the Tyneburn fans was fatally stabbed and subsequent CCTV footage viewed by Trevor, Eddie and Charlie shows that John is responsible.

The following morning John returns to the investigation HQ to find out that the operation is being closed down and later back at New Scotland Yard he is told that the team are to be commended but would now have to serve several weeks of beat police duty at their respected divisions, to which Trevor pleads that if any of his targets see him and John in uniform then they would be killed.

As the group are coming out of the meeting an officer from the licensing department thanks the team for their evidence and tells them that they've been able to use the information in their report to shut down The Rock.

The locations the teams are based on are real – they are neighbouring areas on the northern bank of the River Thames in London's traditional East End.

It is believed that Millwall F.C was the inspiration for one team,[3] as the clubs' fans had a long-standing reputation for violence during the period which the film represents.

[4] AFC Bournemouth, Arsenal, Grimsby Town, Portsmouth and Wimbledon are the only real life teams who are mentioned in the film.

Despite being set in London both interior and exterior shots including the surrounding streets of Rotherham United's stadium Millmoor was used for Shadwell Town's ground "The Kennel".

Other stadiums used in the film were Bradford City's Valley Parade, Crystal Palace's Selhurst Park, Leyton Orient's Brisbane Road and Millwall's old ground The Den.

The sequel saw Perry Fenwick, Lee Ross and Richard Graham reprise their roles from the original film.