I Know What You Did Last Summer by Lois Duncan was originally published in 1973. Duncan was a prolific author, writing more than 50 books but is best known for I Know What You Did Last Summer and Killing Mr. Griffin. After the movie version of the former was released, Duncan updated her novels. Starting in 2010, her edits included changing technology and political events to reflect the new millennium. The publisher added an author Q&A at the end of I Know What You Did Last Summer that addressed the changes. For instance, the original version of I Know What You Did Last Summer includes the impact of the Vietnam War in the 1970s, and the revised edition explores the impact of the Iraq War.

While I Know What You Did Last Summer was adapted into a horror film, the novel is a young adult thriller. The movie includes gore and violence not present in the novel. A clear example of the difference in genre is that none of the characters die in the novel. Both the novel and the movie share the premise of four teenagers being involved in a hit-and-run car accident and the suspense of being stalked almost a year later by a mysterious figure. The novel’s themes include The Effects of Guilt About Manslaughter, Gender Roles, and Family and Identity Formation.

This guide cites the 2020 Revised Trade Paperback edition of I Know What You Did Last Summer.

Content Warning: The novel contains references to the death of a child and depictions of gun violence.

Plot Summary

I Know What You Did Last Summer follows four teens—Julie, Barry, Helen, and Ray—almost a year after a car accident. Last summer—before the novel’s beginning—the teens were partying, and Barry drove Ray’s car while under the influence of alcohol and marijuana. He hit and killed a young boy named Daniel, who was riding his bicycle in the dark. Since Barry was the only 18-year-old of the group, and thus would face much more severe consequences for the hit-and-run, he demands that they keep the accident a secret. Julie votes against this but is outnumbered, and the teens make a secrecy pact.

In the time between the accident and the beginning of the novel—shortly before Memorial Day of the following year—Barry began attending classes at the local university, Helen won a beauty pageant that led to a job at the local television station, Ray traveled around California working odd jobs, and Julie became more focused on her academics. The novel begins with Julie getting her acceptance letter from Smith College alongside an anonymous letter with the phrase from the book’s title: “I KNOW WHAT YOU DID LAST SUMMER” (9).  

Julie reunites with Helen and Barry to tell them about the letter. Barry convinces the girls it is merely a prank, but someone follows his car when he leaves Helen’s apartment. Ray, who recently returned to town—the teens’ hometown is an unnamed town in New Mexico—visits Julie and her mom. He learns that Julie (his former girlfriend) recently started dating a veteran named Bud. Still in a relationship with Barry, Helen meets a new neighbor she calls Collie. At the novel’s end, the reader learns that Bud and Collie are the same person—the one who seeks revenge on the teens for killing Daniel, his half-brother, in the accident.

Helen also receives threatening mail—a picture of a boy on a bicycle cut out from a magazine. Then, Ray receives an old and worn newspaper article covering the accident in the mail. Unlike Barry, he does not think it is a prank and regrets the secrecy pact and losing Julie over it.

On Memorial Day, Barry—who is cheating on Helen—gets a phone call at his frat house. His frat brothers think it is one of the women he is seeing, but the reader later learns that it is a call from Bud/Collie, who says he has evidence that the teens were involved in the accident. This is a bluff, but Barry follows the extortionist’s instructions and goes to the athletic field at the college. During a fireworks display and an anti-war protest, Bud/Collie shoots Barry without it being heard on the field.

After the teens and their families learn about the shooting, Helen tries to visit Barry in the hospital. Collie gives her a ride and accompanies her. Barry’s mom will not let Helen see Barry. Barry makes it through surgery, but there is a chance he might be paralyzed from the bullet. When Barry’s mom continues to keep anyone from seeing Barry, Helen remembers how her sister, Elsa, thought Barry had broken up with her the night of the accident, and she worries that Barry is cheating on her. Collie visits Helen, insisting the shooting was planned and asking her if Barry has any enemies. Helen does not reveal the secret about the accident.

Julie and Ray go to the spot in Cibola National Forest where they partied last summer to discuss the threatening mail. They decide to visit Daniel’s home, the address of which is in the newspaper article that was sent to Ray. There, Julie and Ray meet Megan, Daniel’s sister. She tells them that her mother ended up in a psychiatric hospital in Las Lunas, and her father moved there to be close to her mother. Ray notices men’s clothing drying on lines in the yard and new yellow paint on the house.

After this, Ray and Julie try to convince Helen to break the secrecy pact and tell the police. They do not tell Helen that Barry is cheating on her, and Helen continues to protect Barry, refusing to tell the authorities. Barry, meanwhile, lies to his parents about the phone call, saying Helen called him the night of the shooting. When Julie learns that Helen didn’t make the call, she suspects Elsa made the call and is the antagonist because of the negative interactions she’s had with her. Ray is skeptical. Julie’s mom expresses concern about her, and Julie almost breaks the secrecy pact but does not tell her mom about the accident.

Ray sneaks into Barry’s room in the hospital. Barry is angry that he can’t move his legs and might be paralyzed, unable to play football again. Barry lies to Ray about the phone call, saying it was a woman he was seeing behind Helen’s back and that the shooter stole money out of his wallet. Ray believes Barry.

Ray runs into Bud after leaving the hospital, and they talk over coffee. Ray admits that he is still in love with Julie. Bud bets Ray that Julie won’t go to Smith. Ray thinks this means Bud is trying to marry Julie, but this hints at Bud’s true identity as the antagonist who plans to kill Julie. Ray tells Helen that Barry was robbed, and she believes this story.

Women at Helen’s apartment complex are angry at her for flirting with Collie—taking all his attention—even though she has a boyfriend. Collie invites her to swim with him. After they race, Collie insults her, implying she is self-centered, and he says he has a date with another woman. Helen continues to hang out at the pool after Collie leaves, flirting with other residents. While this is happening, Collie slips into her apartment. When Helen enters, he threatens her, revealing his true identity as the antagonist. Helen hides in her bathroom, eventually jumping out of the second-story window to escape his violent attack.

Bud/Collie then goes to pick Julie up for a date. Julie’s mom warns her that she has a bad premonition and asks her to cancel. Julie reluctantly agrees, but Bud asks her to walk him to his car. Julie reveals his full name—and his dual identity—to the reader. He tells her he is Daniel’s brother and tries to strangle her. However, Ray knocks him out, saving Julie, and tells her that a world without her is the torture Bud/Collie was planning for him.