One day, when I was hiking alone, I was seized by the certainty that I would round the next bend and encounter my parents—not my parents as they are today, in their sixties and living several states away, but in their twenties, not yet married. Would they recognize me? Would I want them to? What would I want them to know? And what of my telling might undo me? I carried the seed of the idea to the Clarion workshop in summer 2016, where instructor Ted Chiang sat us down and—succinctly, without consulting any notes—explained to us precisely how time travel could work, and the issues this poses for free will.