Fourteen-year old Frank Hollahan moves to Florida in 1943, at the height of World War II, to join his father, a navy seaman. When Frank and his mother arrive at the busy naval port of Jacksonville, a surprising new life awaits them. In this new place, Frank's life changes in ways he never imagined. In his new school, his tendency toward exaggeration quickly builds him a reputation as a teller of tales. He wanders to the beach one night and sees what seems to be a man coming ashore from a submarine. When he informs his family, friends, and teachers that he saw a spy from a German U-boat land on the local beach, no one believes him. Is the spy real, or is he only a part of Frank's imagination and exaggeration? Frank is certain the spy has plans for sabotage. With the aid of Rosemarie Twekenberry, who has eyes only for Frank, and a mysterious beach recluse known as Weird Wanda, Frank sets out to prove the spy's existence. With time running out, Frank must figure out a way to stop him. Each rumor and discovery—whether a buried chest, a secret code, or a mysterious note—presents new problems.

The truth finally comes to light at the big bond rally in the shipyard as Frank's class presents a rousing patriotic program, led by Mr. Jolly, an ex-clown turned teacher. Thrown into the mix are a brash, redheaded student named Howard; Gladys, the organizer; and other zany characters who all join in the tangled web of this wartime mystery, based on an actual occurrence. The spy who came in from the sea ends up teaching Frank—and the people of Jacksonville—valuable lessons about friendship, perseverance, and the power of the truth.