The film begins with Reggie Kray stepping out of his East End London apartment, bringing a cup of tea to two policemen who are parked in a car, watching him closely. A voiceover from Frances, Reggie’s love interest, introduces us to one of the policemen, named Nipper Read. This portrayal sets a tone of tension, as Reggie brazenly mocks their surveillance efforts. He soon embarks on a mission to find his missing associate, his driver Frank. Regie’s journey takes him to Frank’s apartment, where he is immediately captivated by Frances, who greets him despite her disapproving mother dismissing him. Driving through the streets, they manage to evade the police tail, leading to a moment of levity when Ronnie Kray, Reggie’s twin brother, shares a bizarre story about willed pudding.

Ronnie, who is serving time for grievous bodily harm but is now in a psychiatric facility, adds a layer of complexity to the Kray family dynamic. Reggie seeks to have Ron declared sane, but the psychiatrist warns that Ron poses a significant danger, prescribing daily medication with a severe warning—“there will be big trouble” if he doesn’t take it. Their bond is further tested when Reggie takes Frances to an East End club he owns, showcasing his dual life—celebrity mingling with dangerous criminals.

As the Krays navigate their turf war with rival gang leaders, we witness the growing chaos in their lives, highlighted by a violent confrontation during a parlay meant for peace. Scrap after scrap reveals the brothers’ loyalty to each other as they resort to brutal violence to secure their position. The Krays’ lawyer, Leslie Payne, emerges as a crucial figure, helping them establish legitimate businesses to mask their deeper illegal undertakings, while Frances narrates that Reggie sees Payne as a key ally, unlike Ron, who detests him.

Despite promising beginnings, Reggie’s legitimate aspirations clash with Ron’s love for crime, leading to friction that culminates in violence within their enterprises. Reggie’s sentence brings further turmoil; while he is imprisoned, Ron’s reckless behavior jeopardizes everything Reggie has worked to build. Reggie’s brief return sees him proposing to Frances, but their marriage soon spirals into despair, exacerbated by Reggie’s continuing criminal involvements that pull him away from home.

Amidst the escalating madness, Frances turns to drugs as her relationship deteriorates, further complicated by Ron’s erratic outbursts and his refusal to take necessary medication. A shocking murder committed by Ron in a crowded room draws the attention of Scotland Yard, wherein Nipper Read takes the lead, vowing to dismantle the Krays’ criminal empire. The police’s frustrations intensify as witnesses are too fearful to come forward, leading to Frances’ tragic decision to leave Reggie, culminating in her heartbreaking suicide.

In a series of misguided decisions, Ron orders a hit on Leslie Payne, which backfires spectacularly, leading to a cascade of legal troubles. Following violent confrontations, including the brutal killing of Jack by Reggie, the police finally gather enough evidence to bring the Krays to justice. The film closes with a graphic telling us about Reggie’s conviction and his eventual death from cancer shortly after a compassionate release, while Ron is left to linger in the psychiatric facility until his passing in 1995. This tumultuous tale encapsulates the perilous struggle between love, loyalty, and the darker aspects of life as a gangster.