The film begins with a summary of how the Allies are struggling to stop U-boats from sinking their freighters. The ”U-571”, has just fired a torpedo and sunk a freighter. The crew’s rejoice is later turned to terror as an Allied destroyer drops several depth charges while the U-571 dives to escape them. Soon a fuel line snaps, which catches fire and kills all the engineering crew. Because of sustained damage, ”U-571” is forced to resurface. With the ”U-571” out of action and no engineers to restore it, an Enigma-encoded SOS is sent to Berlin for aid.

Meanwhile, the crew of the US Navy submarine ”S-33” are celebrating Ensign Larson’s wedding and 48 hours leave. Military police suddenly arrive, announcing a secret mission that ends shore leave…