This is What Happened - Mick Herron

Something's happened. A lot of things have happened. If she could turn back time, she wondered how far she would go.Twenty-six-year-old Maggie Barnes is someone you would never look at twice. Living alone in a month-to-month sublet in London, with no family but an estranged sister, no boyfriend or partner, and not much in the way of friends, Maggie is just the kind of person who could vanish from the face of the earth without anyone taking notice.Or just the kind of person MI5 needs to thwart an international plot that puts all of Britain at risk.Now one young woman has the chance to be a hero - if she can think quickly enough to stay alive.