In September 1943, the Royal Navy launched Operation Source, a top-secret strike against the German warships hiding in the fjords of northern Norway: the cruiser Lutzow, the battleship Scharnhorst, and above all the 40,000-ton battleship Tirpitz. The men who trained for it lived in a gutted Scottish hotel on the Isle of Bute, renamed HMS Varbel, preparing to attack a floating fortress with 50-foot midget submarines known as X-craft.

Drawing on declassified operational reports, technical blueprints, and pilot logs, this episode follows the mission from the intelligence web that found the Tirpitz, including a Soviet pilot photographing the anchorage from 50 meters and the Norwegian Johansen brothers on the ground, through the deadly tow across the sea, the losses of X9 and X8 before the target was even reached, and the final run by X6 and X7 beneath the battleship’s hull. We explain why bombing the Tirpitz from the air would not work, how the crews slipped through anti-submarine nets by tailgating German vessels, and why the underwater explosions crippled the ship for six months without sinking it.

• Why steep fjord walls blocked radar and made the Tirpitz nearly impossible to find or bomb
• The loss of X9 and its three-man passage crew when the tow rope snapped
• How X8’s leaking two-ton charges detonated and forced the crew to scuttle their own sub
• Lieutenants Donald Cameron and Godfrey Place setting the charges and warning Captain Meyer on deck
• The physics of an underwater blast that threw turret Dora off its bearings and earned two Victoria Crosses