In this programme they discover the secret life of the nocturnal otter on British rivers and film unique behaviour of a river otter hunting at night. The boys recreate the river bed in an old tank to investigate the family life of the stickleback. This works brilliantly for the film diary, until all the fish start disappearing, but just who is eating them? Conservationist Philippa launches a search for the fastest declining mammal in the UK, the water vole. What she finds causes both joy and concern. And the cameras reveal what the life of an ordinary mallard mum is really like, as one night she frantically tries to protect her ducklings from being eaten.