Belmarsh prison was built in 1991 to hold Britain's most violent offenders, and this episode looks at a second, less publicized problem inside it: gang warfare over the drug trade. Sixty-four different gangs operate within the prison, carrying outside rivalries onto the wings, and in a four-month stretch in 2021 staff logged nearly 40 gang-related attacks. Officers describe what it takes to keep the peace on wings where enemies are locked up a few doors apart, while former inmates give the inside view. Ex-gang member Bobby Kasanga and former drug addict Daniel Marczewski describe, in blunt detail, the violence and drug dealing that shaped their time at Belmarsh. Interviews with staff and prisoners are cut together with descriptions of specific incidents, building a picture of daily life on a wing where control is constantly contested. The episode stays close to firsthand testimony rather than broad statistics, letting the people who lived it explain how a prison built for security became a battleground for turf.