MI5 (and just about every other agency/department of government) works to foil a terrorist attempt in the UK by an "invisible", i.e. a British citizen. A page-turner with a likeable heroine.
Too many twists and coincidences - it just all got very unbelievable. If the ending is like that I need the reveal to have been foreshadowed somehow, but here I never thought: "oh, I see - I should have realized".