Ian Lightfoot (Tobey Maguire) is a young brave with a particular distaste for mouses. When his brother Hiccup (Tom Cruise) is killed by one, Ian in turn kills the mouse, only to be magically transformed into one himself. To make matters worse, his other brother Barley (Brad Pitt) vows to kill the mouse Ian has become. Ian's only hope is a magical mountain where he believes he can be changed back to a human, and he enlists a real young mouse, Fievel (Thomas Dekker), to get him there.