Christina, Grant, Mimi and Papa fly The Mystery Girl to the Twin Cities to attend the freezing St. Paul Winter Carnival and quickly learn that the Winter Carnival Medallion has been stolen. Christina and Grant team up with a pair of twin sisters to help solve the mystery of the missing medallion. The kids follow a series of clues that will lead them in and out of the Twin Cities including visiting the Mall of America, going on an ice-fishing adventure, and dog sledding through a park.