Scott Calvin has been Santa Claus for the past eight years, and his loyal elves consider him the best Santa ever. But he's got problems, such as a mysterious weight loss, and then he discovers that his son Charlie has landed on this year's "naughty" list. Desperate to help his son, Scott heads back home, putting a substitute Claus in charge of the North Pole. But when the substitute institutes some strange redefinition of naughty and nice that put Christmas at risk, Scott must return with a new bag of magic to save Christmas.