Six hundred and fifty-seven days ago, Meg Kenyon's father left their home in France to fight for the Allies in World War II, and that was the last time Meg saw him. Recently, she heard he was being held prisoner by the Nazis, a terrible sentence from which Meg fears he'll never return. All she has left of him are the codes he placed in a jar for her to decipher, an affectionate game the two of them shared. But the codes are running low, and soon there'll be nothing left of Papa for Meg to hold on to at all.

Suddenly, an impossible chance to save her father falls into Meg's lap. After following a trail of blood in the snow, Meggie finds an injured British spy hiding in her grandmother's barn. Captain Stewart tells her that a family of German refugees must be guided across Nazi-occupied France to neutral Spain, whereupon one of them has promised to free Meg's father. Captain Stewart was meant to take that family on their journey, but too injured to complete the task himself, he offers it to Meg, along with a final code from Papa to help complete the mission - perhaps the most important, and most difficult, riddle she's received yet.

As the Nazis flood Meg's village in fierce pursuit, she accepts the duty and begins the trek across France. Leading strangers through treacherous territory, Meg faces danger and uncertainty at every turn, all the while struggling to crack her father's code.

The message, as she unravels it, reveals secrets costly enough to risk the mission and even her own life. Can Meg solve the puzzle, rescue the family, and save her father?