Living in the small upstate New York town of Milburn (a fictional location which is indicated to be in Broome County east of Binghamton) are four elderly men who are members of a clique called the Chowder Society: John Jaffrey, a doctor; Lewis Benedikt, a retired entrepreneur; Sears James, an attorney; and Ricky Hawthorne, an attorney and James' partner. For the past 50 years these best friends have gathered together and told each other stories and have been great companions. However, their group once consisted of five members. One year earlier Jaffrey had thrown a party at his house in honor of a visiting actress, and their fifth member, Edward Wanderley, had died in an upstairs bedroom during the festivities. There was a look of absolute horror on his face, as if he had been frightened to death.
Ever since that night the friends have been plagued with horrible nightmares, and have taken to telling each other ghost stories. At one of their meetings, Sears tells them a ghost story about when he was a young man. Before deciding to attend law school James had taken a teaching position in a rural community. He developed a fascination with one of his students, a slow, mentally disturbed young boy named Fenny Bate. Fenny and his sister were ostracized by the community, and upon making some inquiries he finds out why. The two children once had an older brother named Gregory, and it was generally believed that Gregory sexually molested his younger brother. The mother of the siblings was dead, their father had abandoned the family, and Gregory was his younger siblings' guardian. One day while repairing a roof Gregory fell off the ladder and was killed, and someone thought they saw the two young Bate children running away from the scene. Sears tells his friends that in time he began to see a threatening young man hanging around the school, and he eventually came to believe it to be the spirit of Gregory Bate. Sears attempted to save Fenny from the clutches of his dead brother, but to no avail. Fenny died, and Sears left the small community when he had finished the school year.
The next morning after telling his story, Sears and Ricky are called out to the farm of one of their clients, who has found some mutilated livestock in his field. Later in the car Sears reveals to Ricky that the previous night's story was not fictitious, but had actually happened to him in his youth. Sears also admits that he is scared, as are all the members of the Chowder Society. They decide to write to Edward's nephew, Donald Wanderley, as Donald had written an occult novel and they think that his research abilities might be employed on their behalf. Before Donald arrives, however, Jaffrey dies in an apparent suicide by jumping off a bridge.
Donald arrives just as the funeral is coming to a close. The three remaining members of the Society tell him that they want him to investigate any possible avenues that he might deem appropriate. Several years previously Donald's brother David had died under mysterious circumstances, and it led him to write his horror novel. Donald tells them the story of what he thinks actually happened. He had landed a teaching position at Berkeley on the strength of a good reception for his first novel and began seeing a beautiful graduate student named Alma Mobley. At first he was inseparable from her, and there was talk of marriage. But over time he began to notice strange things about her and felt that there was something unnatural about Alma. For example, she had a sinister drug-dealing acquaintance named Greg Benton (who was the guardian of a mentally disabled younger brother), even more sinister acquaintances who belonged to a cult associated with the Manson Family, and claimed to be in regular contact with the spirit of a dead man named Tasker Martin who "approved" their relationship. Donald stopped seeing her as much, his work suffered, and one day Alma simply vanished. Upon investigating he discovered that a great many things that Alma had told him about her past were fabrications; for example, she had claimed to be the daughter of a wealthy New Orleans artist named Robert Mobley, who had had two sons but no daughter. In fact, Robert Mobley had once had a bizarre experience somewhat similar to Donald's in which his son Shelby had committed suicide following an affair with a mysterious younger girl named Amy Monckton under the guardianship of a woman named Florence de Peyser who employed a sinister man named Gregorio as her chauffeur. A few months later David called him and told him that he and Amy were engaged, and that he wanted things to be right between Donald and his fiancee. Donald tried to warn David about Alma, but to no avail, and soon afterward David was dead.
Not long after this Lewis Benedikt is murdered in the forest, and Sears and Ricky decide that it is time to tell Donald the most terrible story that the Chowder Society knows — and it,too, is a true tale. Fifty years previously a young woman named Eva Galli had moved to the town. She was in her early twenties and all five of the young men fell head over heels for her, although it was a purely platonic love . One night in 1929—not long after Black Tuesday—Eva came to see them, but she was not acting like herself. She made sexual advances and belittled them. There was a struggle, and Eva fell and hit her head. Believing her to be dead, they conspired to hide the body by putting it in a car and driving it into a deep pond. But at the last moment Eva's body disappeared from the inside of the car, and there was a lynx looking at them from the other bank.
Donald begins his research and quickly comes to the conclusion that what they are dealing with is a manitou, or some other kind of shape-shifting creature. He also believes that Alma Mobley is actually Eva Galli, and also Amy Monckton and the mysterious young actress who was the guest of honor at the party at which his uncle died. He find some of his uncle's tape recordings and listens to parts not even his uncle had heard yet, where she speaks directly to him and the surviving Chowder Society, saying she belongs to an ancient race of beings and that she herself is old enough to remember the first humans in the country.
Donald, Ricky, and Sears are joined in their struggle by Peter Barnes, a young man whose mother was killed by these creatures' servants, previously normal human beings who has been granted new life and powers. Sears is ambushed and killed in his car, and the survivors now realize that the reanimated Gregory and Fenny are helping Eva in her endeavors, and that Gregory is identical to Greg Benton, the drug dealer Donald had encountered in Berkeley, and Fenny was Greg Benton's disabled brother, and Gregory and Robert Mobley's New Orleans acquaintance Gregorio were also one and the same. Gregory tells them that Florence de Peyser helped resurrect him, and that Eva is also subservient to the de Peyser woman. Gregory and Fenny attack Peter, Donald, and Ricky in a movie theater, but they are both killed in the ensuing struggle, leaving Donald to realize that though they have otherworldly powers, the creatures are not truly immortal. The survivors track Eva down and defeat her, but she escapes in a new shape.
Exhausted, Ricky leaves Milburn for an extended vacation with his wife, and Peter prepares for college. Donald keeps watch to see what form Eva will next appear in, and believes it to be the little girl in the opening part of the book.
While in Florida, Eva emerges from the form of the little girl and attempts to twist Donald's mind. He is able to resist, and kills her after she tries to take the form of a wasp to escape. Donald then prepares to go to San Francisco to hunt down the de Peyser woman