Justine Byrne can't trust the people working beside her. She can't trust the women who live down the hall. She can't even trust the man in front of her, and she just might love him.

Inside the walls of Arlington Hall, a former women's college in Virginia that has been taken over by the United States Army, hundreds of men and women sit, bent over stacks of paper. Pencils in hand, they labor to decode countless pieces of communication coming from the Axis powers. Justine works among them, handling the most sensitive secrets of World War II. But she isn't there to decipher German codes. She's there to find a traitor.

It's difficult for Justine to imagine any of the cryptanalysts as a spy. They are some of the most brilliant people in America—and some of the most eccentric—and they understand the stakes: a single secret in enemy hands could end thousands of lives. Someone has decided to share the Allies' secrets anyway. Justine keeps her guard up and her ears open, confiding only in her best friend, Georgette, a fluent speaker of Choctaw who is training to work as a code talker.

Justine tries to befriend each suspect, believing that the key to finding the spy lies not in cryptography but in understanding how code breakers tick. A frightening attack is her first clue that someone is onto her, making it clear that, along with fighting to save her country, Justine is racing a ticking clock counting down the seconds of her own life.