In 1926, Joe Coughlin, a World War I veteran of Irish descent and the prodigal son of Boston police captain Thomas Coughlin, falls in love with Emma Gould, the mistress of notorious gangster Albert White, the boss of the Irish Gang of Boston. Thomas objects to the relationship and advises Joe against his criminal activities, warning him that his wrongdoing will catch up with him eventually. White's rival, Italian Mafia boss Maso Pescatore, finds out about their affair and blackmails Joe to kill White. Joe refuses but he and Emma decide to flee to California. To fund the trip, Joe commits a bank heist with his partner Dion Bartolo, during which three police officers are killed in an ensuing chase. When Joe tries to leave with Emma anyway, she lures him into a trap set by White. Joe is brutally beaten by White and his men and nearly killed before the police arrive and arrest Joe for the policemen's murders..