At the height of the War of Independence, Michael Collins’ squad member Jimmy Mahon is given an intelligence role while Ursula Sweeney, a code-breaker with British intelligence at Dublin Castle, contemplates treason.

In the aftermath of the multiple attacks on General Winter’s spy ring, the British close ranks. There’s a price on Ursula’s head and Jimmy finally confronts the mole inside Collins’ Squad. General Winter commissions Major Mills to take men to Croke Park.

General Winter targets Sinn Féin’s propaganda machine and their finances, while Jimmy puts pressure on Ursula to feed him information.

Jimmy breaks Joey out of prison, and helps Ursula to be reunited with her son. Eithne's outrage at Diarmuid's death is ignored, and Maurice faces an impossible choice.

General Winter suspects Lily of being a spy, and Ursula must try to help her without implicating herself. Robbie plans to kill Eithne, but is stopped by an unexpected opportunity.