Born with two stomachs, Patrick Smash (Bruce Cook) is uncontrollably and devastatingly flatulent. No more than thirty seconds after his birth, he first broke wind, horrifying his parents (Bronagh Gallagher and Victor McGuire) and doctor (Robert Hardy). As he grew up, his gassy emissions became so uncontrollable and destructive that his father had to flee their home, as he was often injured by the power of his son's powerful farts; a force so strong that it can blow people over and curtains open in one mere release. Patrick's unfortunate affliction left him to be a target for bullies. One incident at his school left him particularly humiliated.
An important visitor was addressing the school during assembly and Patrick fought to control his flatulence, but he could not. He passed gas loudly, polluting the hall and disgracing the school. However, while he is physically weak, Patrick finds strength in his disorder and eventually seeks revenge on the school bully Damon (Joshua Herdman) by passing gas in his face, leaving the bully scarred for life. Patrick's only friend is child prodigy Alan A. Allen (Rupert Grint), who has anosmia and therefore, a resilience to Patrick's condition. Alan and Patrick team up, using Alan's intelligence and Patrick's lifelong dream of being an astronaut, to make Thunderpants, reinforced short trousers strong enough to contain Patrick's emissions. Suddenly, Patrick learns that Alan went to America with the government to assist astronauts who are trapped in outer space, and Patrick finds that his condition may be of use to the spacemen in peril.