A detailed look into history, the technical developments and tactics used by both Allied and Axis sides during this long and difficult campaign, featuring such innovations as Asdic, Type 271 radar, Wolfpacks, Cam Fighters, Hedgehog, Huff-Duff, US Blimps, ASV radar, the Leigh light, Metox, Naval H2S radar, Schnorkels and Escort carriers. Contributions from Donald Macintyre, Patrick Beesly, Carl Emmermann, Humphrey de Verd Leigh, Hans Meckel and Bernard Lovell. Although included in video versions of The Secret War as a seventh episode, this last programme was not made as part of the actual series and was aired separately from The Secret War when first shown. The completely different introduction; title music and credits betrays this (particularly as 'The Secret War' does not appear in the title sequence anywhere, unlike the other 6 episodes).