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.

Traumatised by the events of the last 24 hours, PS David Budd falls under suspicion. Pushed to breaking point, David is helped back on his feet by his estranged wife, Vicky, and is determined to play an active role in the investigation.

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.