Criminal kingpin Johnny Bannion is on the eve of his release from prison, on remission for good behaviour, when a stool pigeon, Kelly, is re-imprisoned. At Bannion's behest, Kelly is beaten up by Clobber, an ostensibly amiable but psychopathic Australian prisoner. Mr Barrows, the sinister head prison warder, colludes in this punishment beating, by turning a blind eye.

When Bannion is released from prison next morning, he is met by Mike Carter, a confident, smooth-talking American criminal. While he's been inside, Bannion has been planning the execution of a racetrack robbery that will net £40,000, to be shared out among the gang, including Carter.

To mark Bannion's release, a rowdy drunken party is thrown at his flat that evening, which is interrupted by the sudden appearance of Maggie, Bannion's ex-girlfriend, who launches into a furious, hysterical tirade against him. After the party breaks up soon afterwards, Bannion takes a bath and retires to his bedroom, only to finds a strange woman in his bed. She later reveals herself to be Suzanne, a former friend of Maggie, and seems to be infatuated with the infamous gangster. Bannion is initially resistant to her charms, asking her to leave his flat, but when he senses she is genuine, he ends up asking her to stay the night.

The final arrangements for the robbery are made with Carter and other gang members. Bannion goes to a jewellers in Soho with Suzanne, and arranges to buy her a bracelet. The couple are spotted by Maggie, who is drinking in a café across the road. After Bannion leaves, Maggie goes into the jewellers to confront Suzanne. Maggie finds out that Bannion has arranged to buy a bracelet worth £500, and guesses that he must be planning a heist some time soon.

The robbery at the racetrack takes place without a hitch. Bannion buries the money in a trunk in the middle of a field. In the meantime, Maggie and a mystery man have informed on him to the police and, almost as soon as Bannion return home after the robbery, he is arrested and sent straight back to prison.

In prison, Bannion is unrepentant, arguing that the crime he's committed will be worth a few years imprisonment. But things are no longer going his way in prison: he has an ambivalent but uneasy meeting with Frank Saffron, an Italian gang leader and the 'prison baron', and Mr Barrows puts him in a cell with two Irish roughnecks, in the hope that they will beat him up (in fact, Bannion manages to fight them both off successfully).

Bannion is visited by Suzanne, who he warns to leave his flat and leave London, sensing that she is in danger from other gang members, who may use her as ransom to demand from him the whereabouts of the hidden loot.

A prison riot is engineered in which Kelly deliberately falsely accuses Bannion of unlocking the doors to the warders. Because of fear of reprisals, Bannion has to be transferred to another prison, which gives him the opportunity for escape en route, assisted by Saffron, who has arranged for a getaway car. But Saffron's help comes at a price; it turns out that he is in league with Carter and another higher criminal authority called 'Highgate'. Carter has kidnapped Suzanne and uses her as ransom to make Bannion reveal the location of the money. When it becomes clear that Carter intends to kill Suzanne regardless, Bannion decides to make his escape with Suzanne. But this escape attempt is ultimately unsuccessful and Bannion is shot as he goes to retrieve the hidden money. Carter cradles him as he lies dying in the desolate snowy field, trying one last time to get out of him where the trunk is buried, and two henchmen start digging at a random spot, desperate to find the hidden loot.