After distinguishing himself by courageously neutralising a terrorist threat, troubled war veteran Police Sergeant David Budd of the Metropolitan Police's Royalty and Specialist Protection Branch (RaSP) is assigned as a principal protection officer to the home secretary, the Rt Hon Julia Montague MP.

Another terrorist attack raises the UK threat level to severe. Home secretary Julia Montague holds secret meetings with the director general of the security service, freezing out the police and her own minister of state for counter terrorism.

As the capital shudders in the wake of two terrorist attacks, specialist protection officer PS David Budd is more under pressure than ever to keep the home secretary Julia Montague safe. The threat to her life does nothing to curb Julia's political ambitions, as her counterterrorism bill takes another step towards being made law, provoking political rivalries.

As the security service's position grows weaker, the police takes the opportunity to reclaim control of the investigation. David struggles to come to terms with the devastating events of the previous 24 hours, and finds himself being questioned by counter-terrorism agents. Pushed to breaking point, he finds an unexpected source of support in his estranged wife, who leaves him determined to take an active role in the case - starting by finding out more about those in the government who work closely with Julia.

David investigates Julia's political manoeuvring and finds his fears about a conspiracy at the heart of government could be justified. However, doubts are raised about his psychological health as allegations about his professional conduct come to light.

Principal Protection Officer David Budd has made a major breakthrough in the investigation into the attack on the home secretary, but his enquiries have put him in mortal danger. With the clock ticking and no-one left who believes him, David attempts to prove his innocence. But the evidence against him begins to stack up.