One September morning in 2008, an investment banker approaching forty, his career in collapse and his marriage unraveling, receives a surprise visitor at his West London townhouse. In the disheveled figure of a South Asian male carrying a backpack the banker recognizes a long-lost friend, a mathematics prodigy who disappeared years earlier under mysterious circumstances. The friend has resurfaced to make a confession of unsettling power. In the Light of What We Know takes us on a journey of exhilarating scope-from Kabul to London, New York, Islamabad, Oxford, and Princeton-and explores the great questions of love, belonging, science, and war. It is an age-old story: the friendship of two men and the betrayal of one by the other. The visitor seeks atonement, and the narrator sets out to tell his friend's story but finds himself at the limits of what he can know about the world-and, ultimately, himself. Set against the breaking of nations and beneath the clouds of economic crisis, this surprisingly tender novel chronicles the lives of people carrying unshakable legacies of class and culture as they struggle to tame their futures.