The Château de Chambord rises out of marshy Loire Valley terrain as the strangest building project of the French Renaissance, commissioned by King Francis I and built by 1,800 workmen who hauled some 220,000 tons of stone into a structure with over 400 rooms and 77 staircases. The film traces the persistent theory that Leonardo da Vinci, who spent his final years under Francis's patronage, had a hand in the château's design, particularly its famous double-helix staircase where two spiral flights wind around each other without meeting. Historians and architectural researchers walk through the building's engineering problems, the king's habit of demanding costly last-minute changes, and the runaway expense that nearly bankrupted the crown. A later chapter covers Chambord's unlikely wartime role as a secret refuge for artworks evacuated from the Louvre during World War II, including the Mona Lisa. Modern imaging and structural analysis are used throughout to test old claims against the physical building, weighing genuine evidence against centuries of legend.