Bletchley Park typist Honey Deschamps spends her days at a Type-X machine in Hut 6, transcribing the decrypted signals from the German Army. Halfway across the world, Hitler's armies are marching into Leningrad, leaving a trail of destruction and pillaging the country's most treasured artworks, including the famous Amber Room. As reports begin filtering through about the stolen amber loot, Honey receives a package, addressed to her, carried by a man she has never seen before. He claims his name is Felix Plaidstow and that he works in Hut 3. The package is postmarked from Russia, branded with two censors' stamps. Inside is a small flat piece of amber, and it is just the first of several parcels. Caught between fearing the packages are a trap set by the authorities to test her loyalty or a desperate cry for help, Honey turns to the handsome enigmatic Felix Plaidstow. But then her brother is found beaten to death in nearby woods and suddenly danger is all around.

While working at Bletchley Park transcribing decrypted signals from the German Army as it marches into Leningrad, Honey Deschamps begins to receive mysterious packages from Russia containing small pieces of amber that suggest a coded message.