There is a particular kind of mercy in watching from the outside. Joan Clarke had learned this early, or thought she had—the mercy of observation, of standing at the edge of a thing and seeing it clearly without being consumed by it. She had watched Alan Turing for years, had loved him in the particular way one loves a person who is brilliant and impossible and utterly indifferent to the machinery of social belonging, and she had never required him to know it fully. She had loved him as a frien...