Jill Winter's is traveling on a train to go and stay at her grandmother's while, her mother go on a dangerous journey across the North Atlantic where there is war going on, to see her dying brother .
Jill meets a boy named Quarry, and her, her friend Wendy, and Quarry hang out and listen to the radio while Jill's grandmother is out. The 3 of them become best friends.
Jill finds a carrier pigeon with a message attached to it that says Sonnabend iv. Jill thinks it is a German word but she doesn't know what it means. She knows something is very wrong.
Jill decides to just tell Quarry about the pigeon and he is shocked. They decide they will dig deeper into solving the mystery of who sent the message and why? The first step they would do is to find out what Sonnabend meant.
Jill and Wendy's friendship is ruined after they tried to join the crystals which is a group of kids that hang out together. Jill wonders if something is up with Wendy and her aunt.
Jill spies on Wendy's aunt and Clayton Bishop which is a neighbor, signaling for a German boat. She realizes that they are Nazi sympathizers! She tells the police and the police start searching for them. Jill has saved the town and everyone is overjoyed!