In 1960s East London, identical twin brothers Ronald and Reginald Kray rise through the criminal underworld, making a name for themselves with their gang, The Firm. While Reggie is the charming and strategic mastermind, Ronnie is a volatile and aggressive member of the duo, often causing unpredictability in their operations. Their combined efforts lead to dominance in the East End, as they engage in various illicit activities such as protection rackets, armed robberies, and nightclub ownership, allowing them to intertwine with both legitimate and illicit circles. Reggie's romantic involvement with Frances Shea, a local woman, introduces a layer of complexity to his life, pulling him between his ambitions, personal desires, and loyalty to his unstable brother.

As Reggie's relationship with Frances intensifies, tensions begin to rise between the brothers over their differing visions for their future and the methods by which they control their empire. Growing public and police scrutiny puts pressure on maintaining their feared reputation while managing personal relationships. Reggie must navigate the treacherous landscape of loyalty, love, and power as the bonds of brotherhood are tested against the backdrop of London's criminal underbelly.