How would you summarize The Mystery in the Twin Cities by Carole Marsh?

The Mystery in the Twin Cities is the 42nd book in Carole Marsh’s series for young readers Real Kids, Real Places. The series follows four main characters—Mimi, Papa, Christina, and Grant— as they fly The Mystery Girl airplane to various locations across the United States.
In this book, the family flies to the freezing temperatures of Minneapolis and St. Paul, Minnesota for the St. Paul Winter Carnival. The family sees the Winter Carnival Ice Palace and learns the legend of the Vulcan Krewe and King Boreas’s court. There, the mystery-solving family soon finds out that the Winter Carnival Medallion is gone, presumably stolen. Christina and Grant meet a pair of twins, Jim and Julie, who they team up with to solve the mystery and find the missing medallion. They also meet Mr. and Mrs. Calhoun, a couple who hopes to adopt the twins.
The team of kids begins to follow clues that lead them in and around the Twin Cities area. Their winter adventures include visiting the Mall of America, trying ice-fishing, going dog-sledding through a park, and drinking hot chocolate to stay warm. They try the traditional Scandinavian winter dish, lutefisk, but they are not exactly fans of this fish specialty that is dried and treated with lye. Grant is also in for a winter surprise when he gets stuck to a metal pole.
Over the course of the book, the characters visit Rice Park, the Pioneer Press Building, the St. Paul Hotel, Landmark Center, Minnehaha Falls, and Saint Anthony Falls. They learn about Nordic immigration to Minnesota and the gold rush in that region in the 1800s.