When a handful of British volunteers were pitted against the might of the German battleship Tirpitz in 1943, official records described the raid as the most daring attack of the Second World War. Four-man X-class midget submarines battled for 11 days across arctic seas to the impregnable Norwegian fjord where Germany's biggest ship awaited them. While crew members from two of the vessels survived, mystery has always surrounded the role of the X5's crew. Now a team of divers seeks out the truth in order to reveal the story of the unsung heroes. 2/6.