Billy's got a lot on his mind that he'd rather not think about. So instead, he watches David Attenborough (because Sir David's got all the answers) and swims in the sea, just letting his mind drift... So when new boy and nature fan, Patrick Green, starts at school and becomes Billy's friend, things are different. Whilst swimming out at sea, a mackerel swims up to Billy's face, blows bubbles at him and says: "Fish Boy!" Billy knows he must tell Patrick, because a crazy talking mackerel changes EVERYTHING!