In the peaceful Kinoko Kingdom, a strange pandemic strikes. To find out about the source of the illness, Mario and Luigi set forth on another journey. However, they came across a huge Cooper on the way and are swallowed, can these two save their land? The RPG takes place both within and outside the Cooper, take note of the brothers' specialties as you run around the stages. Sometimes the Cooper's digestive system will help you through. Take another closer look at the Coopers, they may not be that bad after all.