As the capital shudders in the wake of two terrorist attacks, Specialist Protection officer PS David Budd (Richard Madden) is more under pressure than ever to keep the Home Secretary Julia Montague (Keeley Hawes) safe. The threat to her life does nothing to curb Julia's political ambitions as her counter terrorism bill takes another step towards being made law, provoking political rivalries. With the police marginalised from the investigation into the attacks in favour of the Security Service, David is caught between his new-found loyalty to Julia and his superiors' demands for intelligence on her activities.