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...