Scott Calvin has been Santa Claus for the past eight years, and his loyal elves consider him the best Santa ever. But Santa's got problems--he's even mysteriously losing weight--and things quickly go south when he finds out that his son, Charlie, has landed on this year's "naughty" list. Desperate to help his son, Scott heads back home, leaving a substitute Claus to watch over things at the Pole. But when the substitute institutes some strange redefinitions of naughty and nice--putting Christmas at risk--it's up to Scott to return with a new bag of magic to try to save Christmas.