December 1925. Tommy Shelby OBE has acquired unprecedented legitimacy. The former gangster is also a man alone, estranged from his family and focused only on business. But when he receives a mysterious letter on Christmas Eve, Tommy realises that the Shelbys are in danger of annihilation. As the enemy closes in, Tommy flees his country house and returns to the only safe place he knows: Small Heath, Birmingham, the slum where he grew up. Facing a more determined and sophisticated threat than ever before, the Shelby family must find a way to put differences aside, work together, take up arms and fight for survival...