Billy is a lonely boy. He's obsessed with swimming in the sea, which is where he goes to wash his problems far, far away.
Thanks to his mum's mystery illness, his dad has been forced to work extra hours to make ends meet, so Billy locks himself away with David Attenborough films, and ponders the magic of nature.
Meanwhile at school, bullies mercilessly seize on Billy's 'otherness' and make his life as miserable as possible - but then new boy Patrick Green, with 'fingers like steel, strength of a bear', joins Billy's class. And when a mackerel swims up to Billy's face, blows bubbles into his Vista Clear Mask goggles and says: Fish Boy - Billy's whole world changes.