It was one of humankind's most epic quests - a technical problem so complex that it challenged the best minds of its time, a problem so important that the nation that solved it would rule the economy of the world. The problem was navigation by sea—how to know where you were when you sailed beyond the sight of land - establishing your longitude. While the gentry of the 18th Century looked to the stars for the answer, an English clockmaker, John Harrison, toiled for decades to solve the problem. His elegant solution made him an unlikely hero and remains the basis for the most modern forms of navigation in the world today. This film will be both a celebration of Harrison's invention and an adventure story. An expedition on a period sailing vessel as it sails the open sea will demonstrate the life and death importance of finding your longitude at sea.

• SVT Drama
• BBC Horizon
• Richard Dreyfuss
• Patrick Malahide