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

More video evidence of the attack emerges, isolating Carey further from those around her. With pressure building to find the assassin, Carey 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 - has she made a terrible mistake?

Carey is under pressure to charge Whitlock, but with Napier off-radar, she has bigger problems. Pierson's colleagues have a crisis in confidence, which takes a dangerous turn.

Forging an uneasy alliance, 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 has regained control of SO15, but after Figgis makes a bold move, she faces an impossible choice - just how far is she willing to go to protect the truth?