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 eliminating millions of wrong settings, Huff-Duff direction finding on an oscilloscope, the four-rotor Enigma blackout and Operation Drumbeat off the American coast. Then comes the convergence of Black May 1943: centimetric radar powered by the cavity magnetron, the Leigh Light, very long range B-24 Liberators closing the air gap, operations research proving bigger convoys were safer, and Liberty ships outbuilding the losses. Forty-three U-boats died in a single month and Doenitz recalled his packs.

• Why early sonar and cruiser rules left Britain unprepared for surfaced wolf pack attacks
• How the naval Enigma worked and why physical captures were needed to break it
• Huff-Duff and the trap that forced U-boats to transmit their positions
• The technological convergence of centimetric radar, Leigh Lights and long-range aircraft in Black May
• Why one percent losses still terrified London when Britain needed a million tons of imports a week