Il film racconta, come in una cronaca retrospettiva, la cruenta rivalita fra i due pi· grossi "sindacati" del crimine esistenti negli USA ai tempi del proibizionismo. Uno capeggiato da Al Capone, l'altro da Bugs Moran. In quegli anni ci fu una lunga serie di attentati e ½liquidazioni+ da entrambe le parti. Nel tentativo di indebolire Al Capone, Moran fa eliminare un suo potente amico della mafia, servendosi di un traditore. Questa fu la goccia che fece traboccare il vaso. Al Capone si vendic= facendo trucidare sette uomini che avevano partecipato all'omicidio, il 14 febbraio 1929 : la notte di San Valentino.

In the late 1920's Chicago is the crime capital of the world. In 9 years 618 known murders have been committed, none of them officially solved, and gang lords have grossed over $350 million from prohibition and illegal operations. Undisputed king of the underworld is the notorious Al Capone, who sends hoodlums like Peter and Frank Gusenberg to force bar owners into buying his beer instead of that of Bugs Moran, leader of the rival North Side gang. Determined to wrest control from Capone, Moran is planning to slay Patsy Lolardo, head of the dreaded Mafia and a close friend of Capone's. Similarly, Capone has ordered his right-hand man, Jack McGurn, to arrange for the mass murder of Moran and his mob. First to act is Moran, who has the Mafia boss shot down, hoping that his successor will withdraw support from Capone. But Capone quickly evens the score by trapping the potential successor on a train and slashing his throat with a razor. He then puts into motion his plan to get Moran. On the morning of February 14, 1929, some of Capone's gang pose as hijackers and sell a shipment of booze to Moran's hoodlums. The rest of the gang, dressed as policemen, raid the garage where the exchange is being transacted and mow down seven of Moran's men with tommyguns. Capone, who is establishing his alibi in Florida, learns that Moran was not present at the massacre. Though no one is ever brought to trial for the slaughter, the killers all die violent deaths within 22 months; and public indignation eventually effects Capone's downfall.