Peanut butter is the name of the latest invention by Dr. Caver from St. Louis, U.S.A. Emily and Alexander get to know him while visiting their cousin Josephine. And, of course, the three mice are also there when he sets off on a Mississippi steamer on a trip to the World's Fair. Here, his delicious invention is to be presented to the public for the first time. But someone else is desperate to win the first prize there: Professor Waldo, who has invented walnut butter himself. Soon strange things start happening on the steamer, and the mice immediately start investigating.