As the head of GCHQ, Robert Waterman is trained to seek out threats—to find them, and neutralise them, before they become dangerous. When he meets Sara Eden, he doesn't know what to think. His instincts tell him not to trust her. He works with numbers, and data—they don't lie. People do. She's not an agent. But she knows things that not even his most powerful computer systems do. And she can predict events that no one else can see coming. When terror strikes the capital, Robert faces a choice. Trust his instincts? Or put the fate of countless lives in the hands of a stranger...