This is the story of a monumental challenge: finding longitude at sea – determining a ship’s east-west position when far from land. In the 18th century, solving this problem meant global economic power. While others looked to the stars, English clockmaker John Harrison spent decades developing a practical solution. His ingenious invention made him a celebrated hero and is still the foundation of modern navigation. The film is both a tribute to Harrison's creation and an adventure, following a voyage on a historic sailing ship to illustrate the vital importance of knowing one’s longitude.