Part 1

Stephen King's Pet Sematary begins with the Creed family moving into their new two-story colonial house in Ludlow, Maine. Dr. Louis Creed has moved his wife and two children there after accepting a position at the University of Maine medical center. Rachel Creed is apprehensive; her parents, who don't get along with Louis, did not want the family to leave Chicago.

Soon after arriving, five-year-old Ellie scrapes her knee, and one-year-old Gage is stung by a bee. Judson Crandall, their new neighbor, comes to help. Jud and his wife, Norma, are retired and live across the road. They are friendly, and Louis becomes friends with Jud. Louis's father died when he was three, and he now feels a connection with his elderly neighbor. Jud warns the Creeds about the road separating their houses. It is frequented by speeding trucks, claiming the lives of many pets. Jud shows the Creeds a path on the edge of their property that leads to a burial site for dead animals, marked by the scrawled words "pet sematary." Ellie begins to worry about her pet cat, called Church (after Winston Churchill). Louis tries to comfort Ellie, but Rachel does not want him to explain the concept of death to their daughter at such a young age. Rachel was traumatized by her own early experiences with death, as her childhood was spent watching her sister die after a prolonged and painful battle with spinal meningitis.

In the first week of classes, Louis witnesses the death of university student Victor Pascow, who is hit by a car. That night, Louis sees the injured body of Victor standing in his bedroom. With Rachel still asleep next to him, Louis believes he is dreaming and follows Victor outside. Victor leads him to the back of the pet cemetery and warns him to never go into the woods beyond it. When Louis wakes the next morning, he finds dried mud and pine needles in the bed. Still, Louis shakes off the experience as a stress dream.

On Halloween, Norma Crandall suffers a heart attack, but luckily, Louis is able to save her. For Thanksgiving, Rachel takes Ellie and Gage to visit her parents. While they are away, Church is struck by a truck and killed. In his family's absence, Louis is able to put off breaking the sad news to Ellie. Before he tells them, Jud leads Louis to "the real cemetery," an ancient Micmac burial ground past the pet cemetery. According to Jud, whatever is buried in that ground will come back to life. Legend says the Micmacs stopped using the burial site because a Wendigo, a supernatural creature that can give humans a taste for evil with a touch, haunted the land. The next day, Church is resurrected. Ellie does not know what happened, although she stops sleeping with Church in her room because of how he smells. Church, once cheerful, is now rather violent and, according to Louis, "a little dead."

In January, Norma passes away, and Louis helps his friend through the difficult time. Jud arranges for Norma to be buried in Bangor, at the "prettiest cemetery in the world."

Part 2

In part two, Louis suffers in the aftermath of the death of his son, Gage. Rachel and Louis had run after Gage, but could not stop him from running into the road as a truck barreled past. Rachel's father blames Louis for Gage's death. Ellie says she will "pray to God for Gage to come back," and Jud suspects Louis is planning on exhuming Gage's body and burying it at the Micmac burial site. Jud tells Louis about Timmy Baterman, who was killed in World War II but buried in the haunted ground by his father. Timmy appeared to come back to life, but it was really a demon possessing Timmy's body. Timmy's father was forced to kill his son, then killed himself. Jud tells Louis this story in order to warn him of the evil power of the land.

Still, Louis convinces Rachel to take Ellie to Chicago. Rachel wonders if Louis is hiding something, but she agrees to go spend time with her parents if Louis will join them in a few days after finding coverage at work. Ellie feels a sense of dread and tells her parents about foreboding dreams she's been having. She claims she is scared and does not want to go to Chicago. When they land, Ellie is sick and the tells Rachel a dream she has had about the pet cemetery and Victor. Unsettled, Rachel calls Jud, trying to get in touch with Louis. Jud realizes what Louis is up to,and waits for Louis to return home, despite the feeling that the land is telling him to "stay out of this." Meanwhile, Rachel tries to return to Maine.

Louis exhumes his son's corpse and reburies it in the Micmac burial site. He seems compelled by the power of the land, just as the evil power seems to be tampering with Rachel and Jud's attempts to stop Louis. Back home, Louis assembles his doctor's bag with syringes and lethal drugs, just in case he needs to stop something that is not his son. Sure enough, as he is half asleep, something comes into the room and takes a scalpel from the bag.

Part 3

The evil power possessing Gage's body kills Jud and Rachel, who has just arrived back in Maine. Louis kills Church and Gage via lethal injection and sets fire to Jud's house. Steve, however, catches Louis bringing Rachel's body to the Micmac burial ground. Louis claims he waited too long with Gage, but Rachel will be different because she was only recently killed. Steve flees.

After answering questions from the police, Louis waits for his wife. Rachel comes into the house and rests a cold hand on Louis. The book ends with an ominous line:

"Darling," it said.

The use of the word "it" implies that what has returned from the cemetery is not really Rachel—and that evil is still present in Ludlow.