With narration from Paul McGann, this ground-breaking film sets out to solve one of nature's mysterious phenomena: the Bewick swan's dramatic decline. Swans are icons of legend and myth, but they are now at the centre of a great mystery. A pioneering group of scientists and conservationists sets out to discover why we have lost nearly half the Bewick population in the last twenty years. Every year, these majestic birds make one of the world's toughest migrations, across perilous land, sea and skies. Somewhere between the harsh Tundra landscape and the south of England lies the key to their disappearance. We join extreme sportswoman Sacha Dench and award-winning wildlife cameraman Benjamin Sadd, as they follow the swans over 7,000 km, on a journey that pushes both humans and swans to the limits of their endurance. Cutting-edge tracking technology and innovative filming techniques give privileged insight into the birds' hidden world, providing stunning aerial views and the personal stories of swans, Charlotte, Daisy-Clarke and Leho.