A devastating attack puts the new boss of SO15, Commander Rachel Carey, at the heart of a conspiracy more deadly than anything she's encountered before.

More video evidence emerges, isolating Carey further from those around her. With pressure building to find the assassin, she looks for allies in unlikely places.

Carey searches for one suspect, whilst Napier deals with another. As evidence mounts, Carey is forced to avert a further catastrophe.

Carey is under pressure to charge Whitlock as Pierson's colleagues have a crisis in confidence, and matters take a dangerous turn.

Carey and Pierson are pursued through a network of underground tunnels. Pierson's past comes back to haunt him, and the truth about Simon is revealed.

Carey takes control of SO15 and frees Whitlock, forcing Figgis to act. Gill's testimony exposes Garland publicly. Pierson recovers with Wizard's aid. E-Squadron attacks as Carey faces predicted moves.