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?

• Jennifer A. Nielsen is the acclaimed author of the New York Times and USA Today bestselling Ascendance Series: The False Prince, The Runaway King, The Shadow Throne, The Captive Kingdom, andThe Shattered Castle. She also wrote the New York Times bestseller The Traitor's Game and its sequels,The Deceiver's Heart, and The Warrior's Curse; the New York Times bestselling Mark of the Thief trilogy: Mark of the Thief, Rise of the Wolf, and Wrath of the Storm; the standalone fantasy The Scourge; and the critically acclaimed historical thrillers A Night Divided, Resistance, Words on Fire, and Rescue. Jennifer collects old books, loves good theater, and thinks that a quiet afternoon in the mountains makes for a nearly perfect moment. She lives in northern Utah with her her family, and is probably sneaking in a bite of dark chocolate right now. You can visit her online at jennielsen.com or follow her on Twitter and Instagram at @nielsenwriter.