Colin Stafford-Johnson and Lucy Cooke begin a week-long adventure around the UK's wilderness, beginning with a look at the country's rivers. Colin travels up the River Wye following the salmon, while Lucy discovers the wilder side of canals at night. We go behind the scenes with Steve Backshall and the Wild Alaska Live team as they try to film the elusive salmon shark There is also a look at how the return of one of Britain's long-lost beasts, the beaver, is changing the landscape, as well as how the UK's rivers are important for one of the world's most peculiar fish - the sea lamprey.