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  ...