Summary In 2008, an illicit message sent from prison ignites a gang feud in Sheffield and a supposed traitor becomes a marked man. The gang is called the S3 - named after the postcode of their area. Violently territorial the S3 are at war with the rival S4 gang and become locked in a bloody vendetta. By 2009, police had a dossier listing 40 occasions on which the two gangs had opened fire on each other. Over 80 years earlier in the 1920's Sheffield was known as Little Chicago. A gambling gang's brutal beating leaves a man dead and a community reeling as violence tears through the streets of Sheffield. A murder so shocking the police take unprecedented steps and call in a gangbuster. Brothers Gary and Martin Kemp investigate two very different brutal Sheffield gangs, one old and one new, and explore how they have changed and evolved.

• Crime