A forgotten airship. A deadly secret. A woman hunted in the skies above. Summer 1930. On the eve of its daring transatlantic flight to Canada, Britain's newly designed R-100 airship becomes the stage for a deadly game of espionage at a moment when nations are watching closely. Mackenzie Gordon, a trailblazing Scottish naval architect, is abruptly drawn into Britain's R-100 program to design urgent replacement parts for the crippled airship. Thrust into a world of political pressure, national pride, and life-or-death engineering, she must navigate suspicion from superiors who doubt both her youth and her gender. Her father's long-buried past pulls her into a widening web of espionage and betrayal that reaches from the shipyards of Scotland to the shadowed streets of Berlin. As rumors swirl through Glasgow, Mackenzie uncovers a ruthless smuggling network and a dangerous operative known only as Neptune, a double agent whose fractured loyalties threaten to destabilize governments. Urged by a trusted family friend to cross the Atlantic in search of a missing spymaster in Canada, Mackenzie boards the R-100 itself. Aboard the historic airship, she encounters real figures from aviation history, each guarding secrets that could prove fatal. When sabotage strikes mid-voyage and suspicion falls squarely on her, Mackenzie must rely on the skills that made her an engineer, reading stress points, tracing hidden faults, and finding the single weak joint before everything collapses. Double Agent Within is a gripping historical spy thriller grounded in real aviation milestones, blending fact and fiction into a high-stakes tale of courage, deception, and survival in the skies above a changing world.