It is 1942, and 14-year-old Jill is spending the summer with her grandmother in the coastal village of Winter Haven, Maine. The U.S. is at war, and Jill hears rumors that German submarines - U-boats - are just off shore. Trying to ignore her fears, Jill notices suspicious activities in town. When she finds a carrier pigeon bearing the message Sonnabend iv, she's sure that something is wrong. The secret she uncovers will have her running for her life.