Halloween is a few days away when all of the pumpkins in New Mouse City disappear! There's a thief on the loose, and the thief wants to stop Halloween. At the same time, all of the mice in New Mouse City receive an invitation for a free trip to the brand-new Mystery Mouse Theme Park. Geronimo knows something strange is going on, but will he and his detective friend Hercule Poirat be able to solve the mystery in time to save Halloween?