Funella is unusually nervous because her dear Grandmama is coming to visit. Grandmama was the monster who founded the Furchester Hotel, and when she arrives she is unhappy to see that it's not as monstery as it used to be in her day.

Funella and the others catch Monster Pox, a funny illness that only monsters get. It's very contagious and has some very strange symptoms - spots, clucking and disco dancing. As the only non-monster in the hotel, Harvey P Dull is the only one who isn't clucking, dancing and covered in spots. Before he knows it he has to run the hotel by himself and look after all the poorly monsters.