After drifting between school and dead-end jobs, a young man makes the decision to return to the city he left after high school. The magnet is his beloved older sister, Grace: the golden girl, smart and charismatic even when rebelling, and always his hero. Now she is a promising graduate student in science and the center of a group of friends that take "Little Brother" into their fold, where he finds camaraderie, romance, and even a decent job. But it soon becomes clear that all is not well with Grace. She veers into sudden rages, often directed at her seemingly adoring boyfriend, John, who is engaged in the same field of research. Her accusations of betrayal are cryptic, and her brother is especially confused and troubled when she turns on him, accusing him of a fatal disloyalty. A visit to their mother triggers an episode that suggests Grace has tumbled into serious mental illness--except that John seems to know more than he is telling, and some supposedly objective certainties about what is real seem to be starting to fracture. When Grace disappears, the narrator embarks on a mission to discover the truth, a quest that brings him up against an astonishing question: if the universe is infinite, could there be infinite variations of ourselves, past, present, and future, in a dimension only a few can even imagine? And if there are, and we could enter that dimension, what might confront us? And could we ever make it back?