From the introduction:
The year is 1920 and the location for this scenario is Topsfield, Massachusetts. Tired and hungry, an venerable Chthonian selects it's prey for it's final meal. It is none other than Prof. Herbert Cowan, formerly of Miskatonic University. Telepathically bound to his home for several weeks as the creatures draw near, the good professor began to go mad under the creatures influence. Fortunately for Cowan the weary Chthonian found itself within a cavern full of fresh water and passed away. Now free from the creatures hold, the mad man escaped his home ranting and raving about his unseen captor.
All the while the enormous creature rots within the cavern, poisoning the well water of several nearby residents who are dumbfounded by the recent onset of illness.