Over 99 percent of Allied ships bound to and from the British Isles arrived safely, yet Churchill said the U-boat peril was the only thing that truly frightened him. This episode explains the paradox behind the longest continuous campaign of World War II, one that cost the Allies around 3,500 merchant ships and 175 warships and Germany 783 U-boats. It begins with the Admiralty's overconfidence in ASDIC sonar, the blind spots that let a surfaced submarine slip beneath the ping, the naive cruiser rules of interwar treaties, and Karl Doenitz's doctrine of surfaced night attacks by wolf packs that neutralised the British advantage entirely.

From there the story becomes an invisible war of signals. We cover the fall of France and the first happy time, the boarding of U-110 to seize Enigma rotors and bigram tables, Alan Turing's bombes eliminatin ...