The film opens with an unnamed British veteran gangster attending a boxing match with friends. Hearing in conversation that another gangster, Freddie Mays, is to be released from prison after 30 years, he falls silent and leaves the table to recover his composure. The narrative flashes back to the 1960s, showing a younger Gangster. He comes to the attention of an influential London gangster, Freddie Mays, who recruits him as an enforcer. The Gangster is eager to please; his violence impresses Mays and he proves his loyalty with creative methods of murder. However, the Gangster from the start is obsessed with and jealous of Mays' success and glamorous lifestyle, demonstrated in his luxury clothing and plush flat. The Gangster discovers that Mays' rival, Lennie Taylor, is planning to kill Mays. Instead of warning him, the Gangster decides to let the attack take place, and kills the only other member of his gang who is aware of the plan. The Gangster watches as Taylor and his gang shoot and stab Mays, and slit the throat of his fiancee, Karen. Later that night, the Gangster goes to Taylor's flat, shoots him in the leg and tortures him to death. The Gangster discovers the following day that Mays did not die but was hospitalized. Mays is unjustly convicted of ordering Taylor's murder and sent to prison for 30 years. With Mays out of the way, the Gangster assumes leadership of the gang and consolidates his position. In a sequence spanning the years between 1968 and 1999, he is shown organizing a bank heist, opening a casino, fixing horse races, and building his gang to over 300. The narrative returns to the aged Gangster at the boxing event, where he discovers that Karen also survived and is due to marry Mays, who has left prison a changed man. The Gangster summons Mays to his old flat, which the Gangster took over. The Gangster, seeking to resolve any threat and his own demons, offers Mays money and the flat. However, Mays seemingly has no fight left in him, wanting only to marry Karen and retire in peace. The Gangster threatens Mays with a gun, then gives Mays the gun, confesses his silence over the attempted murder and Taylor's death and begs Mays to kill him; Mays leaves, acknowledging how empty and pathetic the Gangster's life is. The film closes with the Gangster, having apparently lost his mind, committing suicide by stepping off the top of a building. His last words: "I'm number one".