The time is June 1942, the beginning of America's first summer at war. Somewhere off the coast of Massachusetts a German U-boat surfaces under cover of darkness. Aboard the U-boat are two agents of the Abwehr, Germany's military intelligence service, whom the ship's commander has been ordered to drop safely on American soil. The agents have as their mission to conduct sabotage against military and industrial installations. Hitler's intention in devising such an operation is to remind Americans he has the capability and the will to bring the war to their backyard.

As the two agents are bringing their equipment ashore, they are surprised by a patrolling US Coastguardsman, who is quickly killed by one of the crewmen from the U-boat but an attack dog accompanying him badly mauls one of the agents. Desperately in need of medical attention, the wounded agent must be returned to the U-boat for treatment. As the crewmen disappear into the night Erich Stinnes, alias Karl Stoner, is left with the sudden realization he will be carrying out this operation on his own.

Stoner is fortunate in not only finding a job at the local shipyard but he is able to rent an apartment from an attractive woman, who is recently widowed. It is not long before the two of them begin an affair that causes Karl, already ambivalent about the war, to question the wisdom of his mission. Before he can resolve his dilemma, an incident at the shipyard exposes him for what he is. Having no choice he is forced to flee leaving Anne to suffer the consequences of her affair with a Nazi spy.