The ‘X’ stood for experimental, but it might equally have meant extraordinary, exotic or extravagant, as this giant submarine attracted superlatives—the world’s largest, most heavily armed, and deepest diving submersible of the day. X.1 was a controversial project conceived behind the backs of politicians, and would remain an unwanted stepchild. This book explores the historical background of submarine cruisers, the personalities involved in X.1’s design and service, the spy drama surrounding her launch, the treason trial of a leading RN submarine commander, the ship’s checkered career, and her political demise. Despite real technical successes, she would finally fall foul of “black propaganda”, aimed at persuading foreign naval powers that the cruiser submarine did not work; even today uninformed opinion repeats the myth of her failure. Well-illustrated with technical drawings and historical photographs, this book pays belated tribute to the real importance of the mysterious X.1.