German submarines and torpedo boats in port with their supply ship.

The U-20 departs from Emden, a seaport in northwest Germany. The two-year-old submarine, propelled by two diesel engines on the surface and by a pair of battery-powered electric motors when underwater, represents a technological advance that already has transformed naval warfare. In the first 10 weeks of the war, Germany's 29 U-boats sank fifty British cruisers, including an engagement in which one submarine sent three warships to the ocean bottom in a matter of minutes.

The U-boat is armed with four torpedo tubes, two in the bow and two in the stern, and a 3.5-inch deck gun. The 35-man crew is commanded by Walther Schwieger, the 30-year-old scion of a noble Berlin family. His orders are to sail to the northern tip of Great Britain, head back down south on the Atlantic side, and then turn east into the channel between Great Britain and Ireland, where the U-20 is to sink ships going to and from Liverpool. Schwieger already has a reputation as a cold-blooded hunter who prefers to attack without warning and has no compunction about firing upon neutral-flag ships that he suspects may be British merchant vessels in disguise.