The series opens as a VX poison gas attack is unleashed on the government and a low level MP is forced to step in as acting Prime Minister. As she strives to establish her place, the heads of MI5 & MI6 are pressing her to make a statement to the press. While acting Prime Minister Katherine Williams struggles with them, MI6 Special Operations Director David Johnson has activated his Black Book Operatives to bring those responsible to justice. Will Black Book Operative John Cain find the terrorists before the police and MI5 do? Will John or MI5's agents work out who ordered the attack?