After her fiancé is killed in the war, 18-year-old June Walker is at loose ends. Wanting to do her bit to help her country, she joins her older sister in Oak Ridge, Tennessee, where a crowd of young women are descending to work at mysterious jobs they're not allowed to talk about. The bustling town—which has suddenly sprung up out of nowhere—is dominated by secrecy and security. Inside its borders, though, Oak Ridge is lively with people bustling in and out of factories, canteens, bowling alleys, movie theaters, and dance halls. June's job as an operator of a big, boring machine is mind-numbingly dull, but her life is more meaningful now than it's ever been. Even though she's not allowed to ask questions about what she's actually doing, she's convinced her role in Oak Ridge—not just as a worker but also as a morale booster for all the young men stationed there—is important.  

Dr. Sam Cantor, a 30-year-old engineer from New York, is charmed by June's country girl naiveté. Sam is one of the few people in Oak Ridge who knows what The Manhattan Project is. As his frowned-upon relationship with the younger woman blooms, it becomes increasingly difficult to keep the secrets he's promised to protect from leakng out. 

Cici Roberts, June's bombshell roommate, is disgusted by the growing closeness between June and Sam, even though she spends her every waking hour trying to snag a rich husband for herself. Desperate to pass as a sophisticated socialite instead of what she truly is, Cici will do anything to keep her own secrets hidden.

Joe Brewer knows that he and his fellow Negro workers are not treated the same as their white counterparts in Oak Ridge. Still, he needs his wages to send back to his wife and children in Alabama. That requires working hard and keeping his head down, keeping far away from the trouble that's brewing in the colored quarters.  

When Hiroshima is bombed and the shocking truth about the true purpose of Oak Ridge is revealed, everyone in the secret city will be left to grapple with their own morality in the wake of the horrifying destruction they've helped to bring about. The lives of June, Sam, Cici, and Joe will be forever changed because of it.