Max is just an ordinary, slightly scruffy boy, until he burps, and that's when he turns into a huge monster, with teeth like swords, who can jump over buildings, roar loudly and eat whole dustbins. It's great being a monster whose aim is to 'protect and do good stuff' until Max sneezes... and when he does he turns back into a boy again, often far from home and dressed only in his pants. But his arch enemy, Peregrine, a boy his age has invented a POOP (Portable Operating Omni Prison) machine to catch Monster Max and put him in a zoo...