Former British intelligence operative Phil Mason has embarked on a new career as a struggling private investigator. He's grateful for the offer of work from his former M15 liaison officer, Joe Lassiter, who requests his assistance in the hunt for members of al-Qaeda cells in the UK. M15 has a shortlist of 'low level' suspects who need to be investigated and eliminated from enquiries. Teaming up with his former associate, Jasmina (Jazz) Alagic, Mason sets out to track down Lassiter's suspects. At first, he finds the surveillance work mind-numbingly dull - until one of the subjects, a man known as Khan, starts to behave extremely suspiciously. Breaking into Khan's home, Mason and Jazz discover him dead, his throat slit. An encrypted email on the dead man's laptop reveals an enigmatic reference to an assassination attempt to be made on the US president during his forthcoming official visit. Heading a small team in the hunt for Khan's killer and the would-be assassins, the terrible truth dawns on Mason: there must be a traitor within the UK security apparatus.

The tension ratchets up as the date of the President's visit fast approaches and the identity of the would-be assassin is revealed: a shocking revelation with dire consequences. Mason must hastily re-learn his old counter-sniper skills if the President's life is to be saved.