This story is about the moment when the world changed forever. It involves disaster at sea and amazing feats of exploration. It all started with one man in North Lincolnshire, but it drew in the Royal Navy, the greatest minds in the land, and the King himself. And at the centre of it all was the clock – the most accurate clock in the world, made by John Harrison, who became obsessed with incredibly accurate clocks, to keep time not on land, but at sea.