In November 1944, eighteen-year-old June Walker boards an unmarked bus, destined for a city that doesn't officially exist. Oak Ridge, Tennessee has sprung up in a matter of months, a town of trailers and segregated houses, 24-hour cafeterias, and constant security checks. There, June joins hundreds of other young girls operating massive machines whose purpose is never explained. They know they are helping to win the war, but must ask no questions and reveal nothing to outsiders.