It is Thomas Shelby's long-awaited wedding day. In the middle of the celebrations, a mysterious visitor imperils the entire Shelby family, and Tommy finds himself pulled into a web of intrigue more lethal than anything he has yet encountered.

Responding to the Italians' actions, Tommy is set on a path of deadly vengeance. Meanwhile, as he makes plans for his mission on behalf of the Russians, he realises there is a traitor in his midst.