Richard Hammond explores the historical, ecological and economic importance of four rivers. He begins by journey along Britain's longest river, the Severn, discovering how it has been a waterway of barriers, division and conflict, but has now become a river of connection. He also sees how its banks were fought over, used as a transport route for hundreds of years, and how it is now being opened back up as a transport route for one of Britain's rarest migratory fish.

Richard journeys down the River Test, which runs for 40 miles through the Hampshire countryside, and discovers why this very rare chalk stream is so important to the environment.

Richard travels to Scotland to journey down the River Clyde and learn about its environmental importance. This wild and powerful waterway is also going through huge environmental changes. From its mouth, Richard goes in search of clawed alien invertebrates who have taken over the upper stretches of the river, decimating wildlife. He sees how the wildness of this river has been harnessed by people to create power, and further downstream he learns about the impact the Clyde has had globally