When Lady Mina Tretheway receives a telegram at boarding school, she doesn't want to read it. In 1918, with war raging, she dreads telegrams, knowing they never bring good news.

At first she doesn't understand the cryptic message. Then she realizes it's written in code, and the message leads her home to Hallington Manor. When Lord Andrew Graham appears with a dashing young American, Lucas, Mina learns that the two of them must work together on a dangerous project for the war effort.

Thinking Mina is just a spoiled aristocrat, Lucas tries to complete the project alone, fearing her inexperience will give them away. But when the project goes very wrong, Mina and Lucas have to find a way to work together to complete the mission before more soldiers disappear into the darkness of war.