Four-thousand-years ago, an ancient people summoned their dark god using a device that centuries later would come to be called a spirit board. When it was over, there was no one left to send it back. Their Oracle Stone was abandoned, the few survivors scattered, and the dangers of the stone and the area in which it was used forgotten.
The life-long obsession of Dr. Ken Richter has been a megalith covered in strange markings. Having finally earned a tenured position at a major university just outside of Boston, Massachusetts, he is finally able to embark on his quest to unravel the mystery behind the stone. The only way to begin is to determine the stone’s most likely place of origin, and Dr. Richter believes that the most promising place to start is a site in North Salem, New Hampshire known to locals as Mystery Hill.
Officially renamed America’s Stonehenge in 1982, the four-thousand-year-old megalithic astronomical complex has an interesting history. Of the many megalithic sites in the northeast, it is the one that offers the most promise as the megalith’s home, due to the fact that during the late nineteenth century, twenty- to sixty per-cent of the stones were removed from the site. In later years, an insurance tycoon and amateur archaeologist made grievous scientific errors in his lust to prove his personal theory, further deepening the mysteries surrounding the site. While the current owners have done their best to fund their own archaeological surveys, only a small portion of the area has been fully explored, and they have been unable to pique the interest of any major university to conduct a thorough survey. Little is known about Mystery Hill, but there is still much more to learn, and Ken believes he is the one who can reveal the mysteries.
Student volunteer Mark Welton spends a night alone at Mystery Hill, watching over the expensive equipment being used to survey the site. Something happens to Mark that night, and although he does not remember exactly what it was, he knows it scared him. He passes the day feeling much like prey: he feels like he is being watched, stalked. That night is a very long one, and the most terrifying of his life. While the implications of doing so could have a negative impact on his future, Mark leaves a message for Dr. Richter, bowing out of the project without explanation. He has no idea that the entity who has lain in wait for four-thousand-years has finally found the perfect victim.
In the weeks to come, Mark is terrorized by a demon, an inhuman entity bent on his destruction. The onslaught continues unabated until his will is finally broken and he can take no more.
It is not easy to convince the Roman Catholic Church to perform an exorcism. They have very specific criteria that must be met in order for them to even consider allowing a parish priest or even a bishop perform the rite. But in some gloomy room of the Vatican and down its shadowy corridors walk men that hope and pray that they will never be called upon to perform the one task for which they spend their entire ordained lives in study and prayer: performing the Roman Ritual of Exorcism. And in this case, the Vatican sends not one, but three. In symbolic representation of the Holy Trinity, the three priests prepare to battle Evil with only their faith to protect them. And they know they need Ken's help to beat this thing and send it, once and for all, back where it came from.